Sets a user function (error_handler) to handle errors in a script. Returns the previously defined error handler (if any), or FALSE on error. This function can be used for defining your own way of handling errors during runtime, for example in applications in which you need to do cleanup of data/files when a critical error happens, or when you need to trigger an error under certain conditions (using trigger_error())
The user function needs to accept 2 parameters: the error code, and a string describing the error. From PHP 4.0.2, an additional 3 optional parameters are supplied: the filename in which the error occurred, the line number in which the error occurred, and the context in which the error occurred (an array that points to the active symbol table at the point the error occurred).
Note: Instead of a function name, an array containing an object reference and a method name can also be supplied.
Note: The following error types cannot be handled with a user defined function: E_ERROR, E_PARSE, E_CORE_ERROR, E_CORE_WARNING, E_COMPILE_ERROR and E_COMPILE_WARNING.
The example below shows the handling of internal exceptions by triggering errors and handling them with a user defined function:
Example 1. Error handling with set_error_handler() and trigger_error()
vector a Array (  => 2  => 3  => foo  => 5.5  => 43.3  => 21.11 ) ---- vector b - a warning (b = log(PI) * a) <b>WARNING</b>  Value at position 2 is not a number, using 0 (zero)<br> Array (  => 2.2894597716988  => 3.4341896575482  => 0  => 6.2960143721717  => 49.566804057279  => 24.165247890281 ) ---- vector c - an error <b>ERROR</b>  Incorrect input vector, array of values expected<br> NULL ---- vector d - fatal error <b>FATAL</b>  log(x) for x <= 0 is undefined, you used: scale = -2.5<br> Fatal error in line 36 of file trigger_error.php, PHP 4.0.2 (Linux)<br> Aborting...<br>
It is important to remember that the standard PHP error handler is completely bypassed. error_reporting() settings will have no effect and your error handler will be called regardless - however you are still able to read the current value of error_reporting and act appropriately. Of particular note is that this value will be 0 if the statement that caused the error was prepended by the @ error-control operator.
Also note that it is your responsibility to die() if necessary. If the error-handler function returns, script execution will continue with the next statement after the one that caused an error.
Note: If errors occur before the script is executed (e.g. on file uploads) the custom error handler cannot be called since it is not registered at that time.