Using Apache with MySQL and PHP to Develop Web Applications
The Context
I had freshly installed CentOS 6.7. I had installed it as a developer workstation to be able to create applications using Linux, Apache, MySQL, and PHP (among other things).
What is LAMP ?
A high-level overview of LAMP's building blocks and overall system environment, displayed here in combination with optionally used web caches.
I was configuring the Apache web server. I wanted to test PHP with MySQL using an old PHP script I wrote back in 2006. To my great disappointment, I've found out that the following command in my PHP script was not working: // Connecting to the database server. $connect = mysql_connect($db_addr, $db_user, $db_pass) or die("Could not connect : " . mysql_error()); All I was getting was a blank page displayed in my user agent (browser). Note that mysql_connect() is a deprecated call. But my PHP script is one that I wrote in 2006.
The Solution
To obtain debug information when running the script, this is one thing you can do:
Edit the file /etc/php.ini
Change the variable display_errors = Off to display_errors = On
Restart the Apache server with: # service httpd restart
To fix my problem, I did the following commands:
# yum install php-mysql
# service httpd restart
And it worked... (although it took me a good amount of time to found this solution).
It seemed that the mysql module was not installed!!!
Figure 1: GRUB 2 menu with cool Apollo 17 background.
Once upon a time we had legacy GRUB, the Grand Unified Linux Bootloader version 0.97. Legacy GRUB had many virtues, but it became old and its developers did yearn for more functionality, and thus did GRUB 2 come into the world.
GRUB 2 is a major rewrite with several significant differences. It boots removable media, and can be configured with an option to enter your system BIOS. It's more complicated to configure with all kinds of scripts to wade through, and instead of having a nice fairly simple /boot/grub/menu.lst file with all configurations in one place, the default is /boot/grub/grub.cfg. Which you don't edit directly, oh no, for this is not for mere humans to touch, but only other scripts. We lowly humans may edit /etc/default/grub, which controls mainly the appearance of the GRUB menu. We may also edit the scripts in /etc/grub.d/. These are the scripts that boot your operating systems, control external applications such as memtest and os_prober, and theming./boot/grub/grub.cfg is built from /etc/default/grub and /etc/grub.d/* when you run the update-grub command, which you must run every time you make changes.
The good news is that the update-grub script is reliable for finding kernels, boot files, and adding all operating systems to your GRUB boot menu, so you don't have to do it manually.
We're going to learn how to fix two of the more common failures. When you boot up your system and it stops at the grub> prompt, that is the full GRUB 2 command shell. That means GRUB 2 started normally and loaded the normal.mod module (and other modules which are located in /boot/grub/[arch]/), but it didn't find your grub.cfg file. If you see grub rescue> that means it couldn't find normal.mod, so it probably couldn't find any of your boot files.
How does this happen? The kernel might have changed drive assignments or you moved your hard drives, you changed some partitions, or installed a new operating system and moved things around. In these scenarios your boot files are still there, but GRUB can't find them. So you can look for your boot files at the GRUB prompt, set their locations, and then boot your system and fix your GRUB configuration.
GRUB 2 Command Shell
The GRUB 2 command shell is just as powerful as the shell in legacy GRUB. You can use it to discover boot images, kernels, and root filesystems. In fact, it gives you complete access to all filesystems on the local machine regardless of permissions or other protections. Which some might consider a security hole, but you know the old Unix dictum: whoever has physical access to the machine owns it.
When you're at the grub> prompt, you have a lot of functionality similar to any command shell such as history and tab-completion. The grub rescue> mode is more limited, with no history and no tab-completion.
If you are practicing on a functioning system, press C when your GRUB boot menu appears to open the GRUB command shell. You can stop the bootup countdown by scrolling up and down your menu entries with the arrow keys. It is safe to experiment at the GRUB command line because nothing you do there is permanent. If you are already staring at the grub> or grub rescue>prompt then you're ready to rock.
The next few commands work with both grub> and grub rescue>. The first command you should run invokes the pager, for paging long command outputs:
grub> set pager=1
There must be no spaces on either side of the equals sign. Now let's do a little exploring. Type ls to list all partitions that GRUB sees:
grub> ls
(hd0) (hd0,msdos2) (hd0,msdos1)
What's all this msdos stuff? That means this system has the old-style MS-DOS partition table, rather than the shiny new Globally Unique Identifiers partition table (GPT). (See Using the New GUID Partition Table in Linux (Goodbye Ancient MBR). If you're running GPT it will say (hd0,gpt1). Now let's snoop. Use the ls command to see what files are on your system:
Hurrah, we have found the root filesystem. You can omit the msdos and gpt labels. If you leave off the slash it will print information about the partition. You can read any file on the system with the cat command:
Reading /etc/issue could be useful on a multi-boot system for identifying your various Linuxes.
Booting From grub>
This is how to set the boot files and boot the system from the grub> prompt. We know from running the ls command that there is a Linux root filesystem on (hd0,1), and you can keep searching until you verify where /boot/grub is. Then run these commands, using your own root partition, kernel, and initrd image:
grub> set root=(hd0,1)
grub> linux /boot/vmlinuz-3.13.0-29-generic root=/dev/sda1
grub> initrd /boot/initrd.img-3.13.0-29-generic
grub> boot
The first line sets the partition that the root filesystem is on. The second line tells GRUB the location of the kernel you want to use. Start typing /boot/vmli, and then use tab-completion to fill in the rest. Type root=/dev/sdX to set the location of the root filesystem. Yes, this seems redundant, but if you leave this out you'll get a kernel panic. How do you know the correct partition? hd0,1 = /dev/sda1. hd1,1 = /dev/sdb1. hd3,2 = /dev/sdd2. I think you can extrapolate the rest.
The third line sets the initrd file, which must be the same version number as the kernel.
The fourth line boots your system.
On some Linux systems the current kernels and initrds are symlinked into the top level of the root filesystem:
$ ls -l /
vmlinuz -> boot/vmlinuz-3.13.0-29-generic
initrd.img -> boot/initrd.img-3.13.0-29-generic
So you could boot from grub> like this:
grub> set root=(hd0,1)
grub> linux /vmlinuz root=/dev/sda1
grub> initrd /initrd.img
grub> boot
Booting From grub-rescue>
If you're in the GRUB rescue shell the commands are different, and you have to load thenormal.mod andlinux.mod modules:
grub rescue> set prefix=(hd0,1)/boot/grub
grub rescue> set root=(hd0,1)
grub rescue> insmod normal
grub rescue> normal
grub rescue> insmod linux
grub rescue> linux /boot/vmlinuz-3.13.0-29-generic root=/dev/sda1
grub rescue> initrd /boot/initrd.img-3.13.0-29-generic
grub rescue> boot
Tab-completion should start working after you load both modules.
Making Permanent Repairs
When you have successfully booted your system, run these commands to fix GRUB permanently:
# update-grub
Generating grub configuration file ...
Found background: /usr/share/images/grub/Apollo_17_The_Last_Moon_Shot_Edit1.tga
Found background image: /usr/share/images/grub/Apollo_17_The_Last_Moon_Shot_Edit1.tga
Found linux image: /boot/vmlinuz-3.13.0-29-generic
Found initrd image: /boot/initrd.img-3.13.0-29-generic
Found linux image: /boot/vmlinuz-3.13.0-27-generic
Found initrd image: /boot/initrd.img-3.13.0-27-generic
Found linux image: /boot/vmlinuz-3.13.0-24-generic
Found initrd image: /boot/initrd.img-3.13.0-24-generic
Found memtest86+ image: /boot/memtest86+.elf
Found memtest86+ image: /boot/memtest86+.bin
done
# grub-install /dev/sda
Installing for i386-pc platform.
Installation finished. No error reported.
When you run grub-install remember you're installing it to the boot sector of your hard drive and not to a partition, so do not use a partition number like /dev/sda1.
Programmer mastering the art of writing excellent, well
commented, and efficient code.
Worked in several small organizations and also for a
multinational enterprise with a larger development team.
Solid experience in 3D computer graphics on Silicon
Graphics workstations.
Expert in geometry and mathematics in general:
linear algebra, differential and integral calculus, discrete mathematics,
statistics and probabilities.
Bilingual,
can speak, read and write in both French and English.
Long experience in SR&ED - Scientific Research and
Experimental Development - Good at trying to solve difficult problems and
writing reports for tax credits.
Good technical writer and communicator.
Enjoy working in a team environment. Can also work
independently.
Love to share wisdom about computers and Internet technologies.
TECHNICAL SKILLS
Applications: CAD/CAM (computer-aided design and
computer-aided manufacturing), Transport, Manufacturing, Accounting, Server‑Client
app., Information Technology
Platforms: UNIX (SGI IRIX, IBM AIX, Linux), Windows,
VAX VMS
RDBMS: Informix, Oracle, IBM DB2,
MySQL, MS SQL Server, MS Access
Tools: MS Office, Photoshop, GIMP, MS
Visual Studio, NetBeans, Eclipse, WebSphere, Struts/Tiles, Movie Maker,
kdenlive, Oracle VM VirtualBox
Google Apps: Docs, Drive, Earth, Search, Maps, Sites, Blogs, Google+
Networking: TCP/IP, UDP/IP, HTTP, HTML, CSS, XML, Apache
and Tomcat web servers, NFS, Samba, DNS, DHCP, FTP, SMTP, JavaMail, SSL (Secure Sockets Layer), WireShark
packet sniffer, iptables firewall, router configuration and port forwarding
PROFESSIONAL EXPERIENCE
MLC - Marc Lemay Consultant, 2006 - present
Freelance consultant performing software development,
troubleshooting, and training.
GIE
Technologies Inc., 2006 - 2010
Engineering Firm Evaluating Infrastructures
Jobs/Tasks: Developing and
documenting C++ applications.
Projects:
ØICS - The
Image Capture System Mobile vision system with FireWire cameras and other
components to survey highway infrastructures.
ØThe Image
Post-Processor A program to convert in JPEG, GIF,
TIFF, and other formats, the raw images acquired by ICS.
ØTLPS - The
Transverse Laser Profiling System A
system of high performance 3D laser profilers that are able to measure complete
transverse road profiles with 1mm resolution at highway speeds to detect
cracks, potholes, and measure ruts.
ØModified/Improved
a Bill of Material (BOM) system implemented in MS Access.
ØTransferred
the BOM data to a new system programmed in COBOL and using a
MS SQL Server.
ØAdded new
reports to the system using Crystal Reports.
The Canadian National
Railway (CN), 1999 -2004
North America's Railroad
Projects:
ØMPS - The
Motive Power (or locomotive management) System
oAdded in MPS the
IC (Illinois Central Railroad) after the CN acquired it.
oDebugged MPS and
added new functionalities to it.
oImproved MPS
database access speed by implementing RPC (Remote Procedure Call) with UNIX
daemons in a client/server model on the WAN.
ØPMRC - The
Performance Monitoring and Rule Compliance Database Loader
Created and documented a super fast application to import data to an
IBM DB2 database running on an IBM AIX server.
Jobs/Tasks:
ØProgramming,
improving, debugging, documenting
ØAnalysing and
proposing improvements to existing systems.
ØKnowledge
transfer to CN IT permanent employees
D.P.
Digital Precision Inc., 1987 - 1999
High Precision Wire EDM Machining Services
Projects:
ØNP3 - The New
Profil Compiler A programming language based on IBM
APT with full geometry computing features.
ØJapax,
Mitsubishi, and Charmilles F45 Post-Processors
ØUtilities for
plotting on Tektronik and HP LaserJet plotters and printers
ØToolpath Generator
for 2-5 axis CNC integrated in the CAD program ICEM/DDN
ØDP Expert
ØMany other
projects: All detailed on 67 pages in my
portfolio if you are interested.
Jobs/Tasks:
ØProgramming/developing
and documenting applications
ØScientific Research and Experimental Development -
Wrote reports for tax credits
ØTraining employees
and knowledge transfer
ØManaged a LAN with
15 computers and 14 CNC
La Maison de l’ordinateur inc., 1983 - 1987
Computer Store Selling Software and Hardware
Jobs/Tasks:
ØSales
representative for Apple Computers
ØPart-time Manager
EDUCATION AND TRAINING
B.Sc.A.
- 1994 University of Quebec in Montreal (UQAM)
Bachelor of
Science in Computer Sciences and IT Management.
D.E.C. - 1987 MontmorencyCollege in Laval
Computer Science Technology
D.E.S. - 1983 College
Laval in Laval
General high school diploma
2005-2006 Training
in Java EE (13 months) at Richard & Associates.
Developed an application using
the WebSphere IDE,
Struts/Tiles, IBM DB2 to store the persistent
data on a server.
Le webdocumentaire Traque interdite, lancé cette semaine, propose une incursion dans l'ensemble des logiciels qui emmagasinent de l'information sur les internautes. On sait que les publicitaires collectent des données sur les internautes, mais pas que les traces que ces derniers laissent sont accessibles à beaucoup plus de personnes que les publicitaires. Matthieu Dugal en discute avec l'une des réalisatrices du webdocumentaire et chercheuse dans le domaine, Sandra Rodriguez. Science et technologie Nos traces laissées sur le web Le samedi 18 avril 2015
Image tirée de la série documentaire Traque interdite avec réalisatrice Sandra Rodriguez Photo : ONF
Lancé récemment, le webdocumentaire Traque interdite fait déjà beaucoup réagir. Produit entre autres par l'Office national du film, il offre à l'internaute de voir ce qu'on lui cache généralement, soit les traces qu'il laisse lorsqu'il navigue. Dans sa chronique « Sous le net » au Téléjournal, Matthieu Dugal explique comment nous sommes constamment épiés sur Internet.
À mi-chemin entre le site web et le documentaire traditionnel, Traque interdite invite l'internaute à suivre le documentariste dans cette expérience afin qu'il puisse voir les traces qu'il laisse derrière lui lorsqu'il navigue sur le web. Il est possible d'être plus anonyme, mais non sans difficulté. Le but de cette traque pour les commerçants est de connaître nos goûts et nos sujets de prédilection pour mieux nous cibler en tant que consommateur. De plus, personne ne sait ce qu'il advient de ces données de masse, ce qu'on appelle le big data. Une chose est certaine, l'enjeu monétaire est important. En 2014, le site YouTube a fait 5 milliards de dollars avec les publicités. Celles publiées sur Facebook rapporteront autant d'ici 2017.
Recently, I have installed and been using Ubuntu. That's a very popular Linux distribution.
You see, I'm an old UNIX geek and I have found out that these new Linux distributions are now much more simpler to install. It is as simple as installing a Windows 7 operating system. This is very good for theordinary personal computer user. They ask you only the language you wish to use, they also want to know your time zone, and finally, you create a user with a password. This user will have administrative privileges, after a successful authentication, when required by some operations.
Though sometimes, you really want to become root, the Super User, to perform modifications in your system. Otherwise, you will need to prefix every administrative commands with sudo and type your password every time administrative credentials are required. This can be a pain if you have several commands to execute as root.
To create a root account with a password, you need to do this once:
Figure 1 - Giving a new password to root
As you can see in the above terminal, you proceed like this:
sudo su
Enter the password for marc in this case - he is a system administrator.
Then, use the command passwd - to change the root password. You can also type passwd root if you want to be more explicit.
Type two times the new password for root.
And voilà!
Note: The passwords are not echoed in the terminal for security reasons.
I have found a very nice article that demystifies the Wi-Fi signal, noise, and quality measurements to better understand the information displayed by monitoring tools such as KSysGuard on my Linux system like we see in this picture:
Picture 1: Wi-Fi Quality, Signal, and Noise Monitoring with KSysGuard, a Linux System Monitor Utility.
The link to the complete article from CNET is written below:
In case the post is removed from the Web, I have copied it here for future reference as a backup measure.
For the most part Wi-Fi technology works quite well to keep your Mac, iPhone, and other devices connected; however, there are times when certain devices or setups may be plagued with Wi-Fi dropouts.
Recently I covered a number of approaches to fixing Wi-Fi dropout problems, though these only covered the configuration options in the router and the computer itself. Besides the system configuration, an cause for Wi-Fi drop-outs can also be poor Wi-Fi signal quality.
When Wi-Fi drop-outs happen, you may see one of these symbols in the OS X menu bar.Apple
The quality for any analog signal, be it electrical, optical, or radio, can be determined by comparing the desired signal level to the background noise level in the signal in what's known as a signal-to-noise ratio.
In all analog connections you have a certain amount of unwanted signal called "noise," which is a combination of interference from other electrical devices and the innate "fuzziness" of the output. For example, if you turn on a cheap stereo player and crank up the volume without playing anything, you will likely hear a bit of static "white noise" in the background. This innate noise in the signal comes from the quality of the electronics in the player itself. In addition to this noise, you may hear a humming or buzzing sound, which is interference from other electrical devices that is being picked up by the player's electronics.
When you play music (a desired signal), this background noise is usually drowned out because the signal for the music is far greater than the unwanted noise signals; however, this is not always the case. If you are listening to music over the radio, then static and interference from other radio waves can at times be so great that you cannot make sense of the desired audio signal from the noise. In this case either the desired signal is too weak, or the noise signals are too strong and the radio cannot filter them out or otherwise manage them, resulting you hearing a garbled mess.
These examples deal with audio, but Wi-Fi waves work in a similar manner, where background noise can result in the inability for the computer to understand the desired signal, especially when the signal is weak. Therefore, to determine the Wi-Fi signal quality you need to be able to compare the data and noise signals that your computer's radio is picking up.
The Wi-Fi receiver on your computer is constantly measuring both the desired signal strength and the noise signal strength, and if you are experiencing connectivity problems, you can use a tool that will display these measurements for you. On Mac systems running OS X Lion, Apple has included a program called Wi-Fi Diagnostics Utility (available in the /System/Library/CoreServices/ folder) that can do this, but if you have other versions of OS X, then you can use a third-party tool such as APGrapher, AirRadar, iStumbler, or Kismac.
Moving a computer near the router will give a much higher signal-to-noise ratio by increasing the received signal strength, even though the noise does not change.Screenshot by Topher Kessler
In the tool you can monitor the Wi-Fi performance, and see the graphed signal and noise power levels, which in most cases will be negative numbers. This may seem odd, but is correct because the measurement of the wireless signal in watts is converted to a logarithmic ratio unit called dBm, which is the ratio of the measured signal to one milliwatt of power. If the signal is greater than one milliwatt, the dBm measurement will be positive, and if the signal is less than one milliwatt, then the ratio will be negative (logarithms of values between 0 and 1 are negative).
For most Wi-Fi networks, you will see the signal measurement be between around -10 and -70 dBm, and should see the noise between -80 to -100 dBm. In these, the more negative the number is, the smaller its signal is.
The overall power of either the signal or the noise level does not matter; instead, what matters is the ratio of these two. If the desired signal is too weak, then it will fall into the noise and make it hard to distinguish. On the other hand, if the noise level gets too large, then it can also encroach on the desired signal level and likewise make it harder to distinguish. In both cases the ratio of signal-to-noise level gets smaller and the quality of the signal goes down.
What happens when signal to noise ratio is low?
Interference or weak signal strength can cause a low signal-to-noise ratio, resulting in slow connections or dropped connections (arrows).Screenshot by Topher Kessler
Digital data such as that sent over a Wi-Fi signal is sent in packets, each of which is checked for integrity and assembled with other received packets to complete the data stream. This process ensures the data is kept intact; however, it does not overcome the fact that the digital data must always be transmitted over a physical analog signal (air, light, electromagnetism, etc.).
If the physical analog signal carrying the data degrades in quality, then the computer listening to the signal will have a harder time receiving intact packets that it can understand, and the system will spend a lot of time discarding incomplete packets and waiting for them to be resent over the poor-quality analog signal.
Therefore, as the analog Wi-Fi signal quality degrades, the first thing that happens is the speed of the connection drops since the system spends more time asking for duplicate packets than it spends steadily receiving them. As the signal quality degrades further, the system will have a much more difficult time maintaining other aspects of the connection than just the data stream, and you will begin to see the computer lose its handshake with the router, resulting in a dropped connection.
If the signal quality is low (left), the speed of the connection will be much slower. As the quality improves, the connection speed will increase.Topher Kessler
How do you fix a low signal-to-noise ratio?
The approach to fixing Wi-Fi signal quality problems depends on which aspect of the signal-to-noise ratio is not in its expected range. If your measurement of the signal power levels shows the "Signal" component being relatively low (around -70dBm), then you will need to find a way to boost this signal, which can normally be done with one of these methods:
Move closer to the source Signal levels will attenuate more the further you get from the source of the signal, so try moving closer to your Wi-Fi router to see if the signal level increases.
Increase the radio power Many routers have an option for adjusting the Wi-Fi signal level, so consult your router manual to see about increasing this level. Not only will doing this increase your router's range, but it will also increase the quality of the signal and therefore increase your average connection speed (with the router, and not necessarily the internet).
Remove obstructions The signal from the router may be grounded by large metal devices between your computer and the router. Therefore, if your computer is situated in an area with obstructions between you and the router, then try moving to an area where you have a clear path to the router. Sometimes piping or electrical wiring in walls can be enough to ground and attenuate the signal being sent to your system.
Sudden energy pulses from appliances can cause large noise spikes. In this case a refrigerator turning on results in a large noise spike that can cause a data stream pause even if the connection does not drop.Screenshot by Topher Kessler
If you find that the signal is high (-60 to -10dBm), but the measured noise level is also high (above around -75dBm), then in addition to the three suggestions mentioned above try checking for any active electronics around either your computer or the router, and remove them. Large appliances can generate massive amounts of electromagnetic energy that can disrupt the low-power signals in Wi-Fi connections. Even if the energy from appliances does not cause steady interference, it can result in periodic spikes that can cause a connection dropout, especially when the appliance is turned on or off.
Beyond managing the physics of the Wi-Fi connection, you can also many times address connectivity problems in software. If there are firmware updates available for either your router or the Wi-Fi adapter in your system (Apple releases these as EFI Firmware updates), then apply them, as they can sometimes fix a poorly managed hardware device that results in degraded signal quality. In addition to firmware updates, check for software driver or operating system updates for your system, since these can affect how the system interprets the power levels measured in the Wi-Fi adapter.
