Debugging techniques for PHP programmers by Tyler Anderson

Introduction

There are many PHP debugging techniques that can save you countless hours when coding. An effective but basic debugging technique is to simply turn on error reporting. Another slightly more advanced technique involves using print statements, which can help pinpoint more elusive bugs by displaying what is actually going onto the screen. PHPEclipse is a Eclipse plug-in that can highlight common syntax errors and can be used in conjunction with a debugger to set breakpoints.

Setting up

To learn the concepts described in this article, you are going to need PHP, a Web server, and Eclipse. The latest version of PHP supported by the debugger extension is V5.0.3.

We need a Web server to parse the pages you create in PHP and display them to the browser. This article uses Apache2. However, any Web server will suffice.

To take advantage of some of the debugging techniques in this article, you need to install Eclipse V3.1.1 and the plug-in: PHPEclipse V1.1.8. Since Eclipse requires Java™ technology, you also need to download that.

You also need the debugger module extension for PHP. Installing it is a bit tricky. Carefully follow the instructions for installing the debugger extension. For now, comment out the lines where you are asked to load and configure the extension in PHP in the php.ini file. We’ll uncomment those lines when we’re ready to use the debugger.

See Resources for download information. Now let's move on to error messages.

Error messages

Error messages are your first line of defense as a developer. You don't want to be developing code in PHP on a server that is not configured to display error messages. However, keep in mind that when your code is debugged and ready to go live, you want to make sure error reporting is turned off because you don't want visitors to your site seeing error messages that may give them enough knowledge to exploit a weakness and hack your site.

You can also use error messages to your advantage because they display the exact line of code that threw or generated an error. This makes debugging a matter of looking at the line number shown on the browser by the generated error and checking that line number in your code. Later, you will see that the PHPEclipse plug-in aides significantly in the development and debugging process by underlining syntax errors on the fly and by marking syntax errors with a red "x" when saving your file.

Let's take a look at how to turn error reporting on in the php.ini file and set the level of error reporting. Then you'll learn how to override these settings in the Apache configuration file.

Error reporting in PHP

There are many configuration settings in the php.ini file. You should already have set up up your php.ini file and placed it in the appropriate directory, as shown in the instructions in the Install PHP and Apache2 on Linux document (see Resources). There are a couple configuration variables you should know about when debugging your PHP applications. Here they are with their default values:

display_errors = Off error_reporting = E_ALL

You can discover the current default values of these variables by searching for them in the php.ini file. The purpose of the display_errors variable is self-evident -- it tells PHP whether or not to display errors. The default value is Off. To make your life easier in the development process, however, set this value to On by replacing Off:

display_errors = On

The error_reporting variable has a default value of E_ALL. This displays everything from bad coding practices to harmless notices to errors. E_ALL is a little too picky for my liking in the development process because it clutters the browser output by displaying notices on the screen for small things like uninitialized variables. I prefer to see the errors, any bad coding practices, but not the harmless notices. Therefore, replace the default value of error_reporting as follows:

error_reporting = E_ALL & ~E_NOTICE

Restart Apache, and you're all set. Next, you'll learn how to do the same thing on Apache.

Error reporting in the server

Depending on what Apache is doing, turning error reporting on in PHP may not work because you may have multiple PHP versions on your computer. It's sometimes hard to tell which PHP version Apache is pointing to because Apache can only look at one php.ini file. Not knowing which php.ini file Apache is using to configure itself is a security problem. However, there is a way to configure PHP variables in Apache to guarantee the setting of the correct error levels.

Also, it's good to know how to set these configuration variables on the server side to veto or pre-empt the php.ini file, providing a greater level of security.

You should already have toyed with basic configurations in the http.conf file at /conf/httpd.conf when you configured Apache.

To do the same as you just did in the php.ini file, add the following lines to your httpd.conf to override any and all php.ini files:

php_flag display_errors on php_value error_reporting 2039

This overrides the flag you have set for display_errors in the php.ini file, as well as the value of error_reporting. The value 2039 stands for E_ALL & ~E_NOTICE. If you prefer E_ALL, set the value to 2047, instead. Again, make sure you restart Apache.

Next, we'll test error reporting on your server.

Testing error reporting

You will save a great deal of time if you leave error reporting enabled. Errors in PHP point you right to the error in your code. Create a simple PHP file, test.php, and define it as shown in Listing 1.

Listing 1. A simple PHP that generates an error

print("The next line generates an error. "); printaline("PLEASE?"); print("This will not be displayed due to the above error."); ?>

The first print() statement should display its contents to the Web browser. However, the second statement generates and displays an error to the Web page. This results in the last print() statement do nothing