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Debugging PHP

  • September 30, 2002
  • By Elizabeth Fulghum
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PHP's Error Handling Features

In the development phase of creating an application, having notices and errors displayed in the browser is vital. When the project is live, though, displaying errors to users is frequently undesirable, especially since error messages can contain run-time information about the server and script. Depending on what it is, this information can be a boon to potential hackers. This is just one of the reasons that PHP provides flexible options for controlling when, how, and which errors messages are displayed.

As you may have noticed from the sample error messages in the previous section, PHP divides syntax errors into three separate groups: Parse Errors, Notices and Warnings. Parse Errors are classified irrecoverable errors which stop the execution of the script, while Notices and Warnings are less serious and do not stop the script.

To allow the internal settings and functions to control how errors are reported, PHP assigns constants and values to each of the various types of errrors. The following have been available since PHP3:

1
E_ERROR Irrecoverable Error (script terminates)
2 E_WARNING Run time Error (script will not terminate)
4 E_PARSE Parse errors at the time of complition
8 E_NOTICE Notices, less serious than a warning.
  E_ALL All error messages.

And these were added in PHP4:

16
E_CORE_ERROR
A fatal error during PHP's startup.
32 E_CORE_WARNING Warning during PHP's initial startup.
64 E_COMPILE_ERROR Irrecoverable error during compliation
128 E_COMPILE_WARNING Warning during compilation
256 E_USER_ERROR User error message
512 E_USER_WARNING User warning
1024 E_USER_NOTICE User notice

The PHP.ini is the primary location for controlling error reporting options and contains several settings to manage them; including error_reporting, display_errors, and error_log.

The first, error_reporting, uses the constants listed above, and several logical operators ("|","~", "!" and "&") to build up an expression describing which errors should be displayed. For example, the default setting in PHP4 is: E_ALL & ~E_NOTICE. This indicates that all error messages should be shown, except for notices.

When set to "off", the display_errors suppresses all Parse Errors, Warnings, and Notices that would normally be sent to the browser. This setting works in conjunction with error_log, which allows you to specify that errors should be saved to the server's error log.

Errors reporting settings can also be set on a script-by-script basis using the error_reporting() function. The rules defined by this function are only in affect for as long as the script runs. This is especially useful in situations where you may want to display errors in a particular script for development and debugging, but suppress them once the script is ready to go live without making global changes in your PHP.ini. error_reporting() accepts constant names (the encouraged method) in combination, as in the PHP.ini, as well as their integer values:

error_reporting(0); - turns off all error reporting.
error_reporting(E_ALL); - turns on all error reporting.
error_reporting(E_ERROR | E_WARNING); - shows critical errors and warnings only.

You can also suppress errors generated by a particular expression within a script by including the proceeding it with the @ symbol. For example: @unlink($file);

Keep in mind that even if you supress errors through any of the above methods, a script will still stop executing should the PHP processor encounter a critical error.

A Note on Creating Your Own Error Messages

You may have noticed that some of the error constants in the previous section referred to user created errors. Though beyond the scope of this article (and the needs of most new PHP programmers), keep in mind that PHP also provides advanced functions which allow you to create and impliment your own error messages. For simple scripts the need to add custom error handling is questionable, but in full-fledged applications the benefits become more apparent. By creating your own error handling functions, you can control the output; for example, you could provide contact information for the adminstrator of the server, or redirect the user to another location. You could also send out an email to notify you if a critical error had occured.

The set_error_handler() function allows you to build in this functionability to your own scripts.

Finally

Hopefully, this article has armed you with the knowledge to intelligently manage and combat errors in your own scripts. Avoiding, and understanding how to quickly troubleshoot errors in your work will greatly decrease the time it takes to get a script from the development phase to afinished product.

In the next article, we will be taking a look at some more theory of programming with PHP, covering aspects of coding style.

Stay Tuned!

Things to remember:

  • Generally, there are two types of errors: syntax errors and logical errors.
  • You can help prevent logical errors by outlining your project with pseudo code before you begin
  • Syntax errors are frequently accidental, and thus more difficult to prevent.
  • PHP has core error handling features that allow you to control error message output.
  • For the purposes of error reporting, PHP divides syntax errors into three groups: Errors, warnings, and notices.
  • Error handling can be set up in the PHP.ini and in the running script.
  • @ before an expression in a script supresses any errors, notices or warnings that might otherwise appear.

Liz Fulghum currently lives in Annapolis, MD where she works as a web designer for a custom shirt retailer. Liz was convinced to try PHP as an alternative to Perl; she has been a fan ever since and frequently works as a freelance developer.

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