int error_reporting ([ int $level ] )
The error_reporting() function sets the error_reporting directive at runtime. PHP has many levels of errors, using this function sets that level for the duration (runtime) of your script. If the optional level is not set, error_reporting() will just return the current error reporng level.

level
The new error_reporting level. It takes on either a bitmask, or named constants. Using named constants is strongly encouraged to ensure compatibility for future versions. As error levels are added, the range of integers increases, so older integer-based error levels will not always behave as expected.

The available error level constants and the actual meanings of these error levels are described in the predefined constants.

Returns the old error_reporting level or the current level if no level parameter is given.

Version Description
5.4.0 E_STRICT became part of E_ALL.
5.3.0 E_DEPRECATED and E_USER_DEPRECATED introduced.
5.2.0 E_RECOVERABLE_ERROR introduced.
5.0.0 E_STRICT introduced (not part of E_ALL).

Example #1 error_reporting() examples

<?php

// Turn off all error reporting
error_reporting(0);

// Report simple running errors
error_reporting(E_ERROR | E_WARNING | E_PARSE);

// Reporting E_NOTICE can be good too (to report uninitialized
// variables or catch variable name misspellings ...)
error_reporting(E_ERROR | E_WARNING | E_PARSE | E_NOTICE);

// Report all errors except E_NOTICE
error_reporting(E_ALL & ~E_NOTICE);

// Report all PHP errors (see changelog)
error_reporting(E_ALL);

// Report all PHP errors
error_reporting(-1);

// Same as error_reporting(E_ALL);
ini_set('error_reporting', E_ALL);

?>

Warning
Most of E_STRICT errors are evaluated at the compile time thus such errors are not reported in the file where error_reporting is enhanced to include E_STRICT errors (and vice versa).
Tip
Passing in the value -1 will show every possible error, even when new levels and constants are added in future PHP versions. The E_ALL constant also behaves this way as of PHP 5.4.

The display_errors directive
The html_errors directive
The xmlrpc_errors directive
ini_set() - Sets the value of a configuration option