Debug PHP applications: How to get useful errror messages

For syntax errors, you need to enable error display in the php.ini. By default these are turned off because you don't want a "customer" seeing the error messages. Check this page in the PHP documentation for information on the 2 directives: error_reporting and display_errors. display_errors is probably the one you want to change. If you can't modify the php.ini, you can also add the following lines to an .htaccess file:

php_flag  display_errors        on
php_value error_reporting       2039

You may want to consider using the value of E_ALL (as mentioned by Gumbo) for your version of PHP for error_reporting to get all of the errors. more info

3 other items: (1) You can check the error log file as it will have all of the errors (unless logging has been disabled). (2) Adding the following 2 lines will help you debug errors that are not syntax errors:

error_reporting(-1);
ini_set('display_errors', 'On');

(3) Another option is to use an editor that checks for errors why you type, such as PhpEd. PhpEd also comes with a debugger which can provide more detailed information. (The PhpEd debugger is very similar to xdebug and integrates directly into the editor so you use 1 program to do everything.)

Cartman's link is also very good: http://www.ibm.com/developerworks/library/os-debug/

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PHP Most common errors

This post is related to the common issues we can have when develop in PHP and the way to debug into each one of them:

Nothing is seen. The page is empty and white.
Code doesn't run/what looks like parts of my PHP code are output
Warning: Cannot modify header information - headers already sent
Warning: mysql_fetch_array() expects parameter 1 to be resource, boolean given
Warning: [function] expects parameter 1 to be resource, boolean given
Warning: [function]: failed to open stream:
Warning: open_basedir restriction in effect
Warning: Division by zero
Parse error: syntax error, unexpected '['
Parse error: syntax error, unexpected T_XXX
Parse error: syntax error, unexpected T_ENCAPSED_AND_WHITESPACE
Parse error: syntax error, unexpected T_VARIABLE
Fatal error: Allowed memory size of XXX bytes exhausted (tried to allocate XXX bytes)
Fatal error: Call to a member function ... on a non-object
Fatal Error: Call to Undefined function XXX
Fatal Error: Cannot redeclare XXX
Fatal error: Can't use function return value in write context
Fatal error: Using $this when not in object context
Notice: Undefined variable
Notice: Undefined Index
Notice: Uninitialized string offset: X
Notice: Use of undefined constant XXX - assumed 'XXX'
Notice: Array to string conversion
MySQL: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near

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