Part Two: PHP Debugging Basics

We assume that you have some knowlege of programming in PHP. Basics to debug your code are summarised here.

Configuring Error Reporting

To make error messages display in the browser, set the display_errors configuration directive to On. To send errors to the web server error log, set log_errors to On. You can set them both to On if you want error messages in both places.

An error message that the PHP interpreter generates falls into one of five different categories:

Parse error: A problem with the syntax of your program, such as leaving a semicolon off of the end of a statement. The interpreter stops running your program when it encounters a parse error.
Fatal error: A severe problem with the content of your program, such as calling a function that hasn't been defined. The interpreter stops running your program when it encounters a fatal error.
Warning: An advisory from the interpreter that something is fishy in your program, but the interpreter can keep going. Using the wrong number of arguments when you call a function causes a warning.
Notice: A tip from the PHP interpreter, playing the role of Miss Manners. For example, printing a variable without first initializing it to some value generates a notice.
Strict notice: An admonishment from the PHP interpreter about your coding style. Most of these have to do with esoteric features that changed between PHP 4 and PHP 5, so you're not likely to run into them too much.

Fixing Parse Errors
The first time you write a PHP program, you discover that the PHP interpreter is extremely picky. If you leave out a necessary semicolon or start a string with a single quote but end it with a double quote, the interpreter doesn't run your program. It throws up its (virtual) hands, complains about a parse error, and leaves you stuck in the debugging wilderness.

Another feature of these editors is quote and bracket matching. This helps to make sure that your quotes and brackets are balanced. When you type a closing delimiter such as }, the editor highlights the opening { that it matches. Different editors do this in different ways, but typical methods are to flash the cursor at the location of the opening {, or bold the { } pair for a short time. This behavior is helpful for pairs of punctuation marks that go together: single and double quotes that delimit strings, parentheses, square brackets, and curly braces.

Once you've got error reporting set up as you like it, and you know how to find parse errors, and you can inspect program data, you're on your way to a fruitful debugging career. However, a fully fleshed-out PHP programmer's toolbox consists of much more than just the tips in this article.