PHP - Error Handling

PHP provides functions for error handling. They allow us to define our own error handling rules. This also allows us to change and enhance error reporting to suit our needs. The error reporting functions allow us to customize what level and kind of error feedback is given, ranging from simple notices to customized functions returned during errors.

Defining Custom Error Handling Function

The PHP set_error_handler() function is used to set a user-defined error function to handle errors in a script.

This function is used to define the way of handling errors during runtime by user, 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.

The standard PHP error handler is completely bypassed for the error types specified by error_levels unless the callback function returns false. The user-defined error handler must terminate the script, die(), if necessary. error_reporting() settings will have no effect on this function and this function will be called regardless - however the user will still be able to read the current value of error_reporting and act appropriately.

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, E_COMPILE_WARNING independent of where they were raised, and most of E_STRICT raised in the file where this function is called.

If errors occur before the script is executed the custom error handler cannot be called since it is not registered at that time.

Syntax

set_error_handler(callback, error_levels)

Parameters

handler(errno, errstr, errfile, errline)

Return Value

Returns the previously defined error handler (if any). If the built-in error handler is used null is returned. If the previous error handler was a class method, this function will return an indexed array with the class and the method name.

Example: set_error_handler() example

The example below shows the usage of set_error_handler() function.

<?php
//a user-defined error handler function
function myErrorHandler($errno, $errstr, $errfile, $errline) {
  echo "<b>My ERROR</b> [$errno] $errstr<br>\n";
  echo "Error on line $errline in file $errfile<br>\n";
  echo "Aborting...<br>\n";
}

//setting user-defined error handler function
set_error_handler("myErrorHandler");

$test = 100;

//triggering user-defined error handler function
if ($test==100) {
  trigger_error("A custom error has been triggered");
}
?>

The output of the above code will be:

My ERROR [1024] A custom error has been triggered
Error on line 20 in file index.php
Aborting...

Defining Custom Exception Handling Function

The PHP set_exception_handler() function is used to set a user-defined exception handler function. The script will stop executing after the exception handler is called.

Syntax

set_exception_handler(callback)

Parameters

handler(Exception $ex): void

handler(Throwable $ex): void

Return Value

Returns the previously defined exception handler, or null on error. If no previous handler was defined, null is also returned.

Example: set_exception_handler() example

The example below shows the usage of set_exception_handler() function.

<?php
//two user-defined exception handler functions
function myException($e) {
  echo "Exception: ".$e->getMessage();
}

//sets user-defined exception handler function
set_exception_handler("myException");

//throw exception
throw new Exception("Uncaught exception occurred!");
echo "This will not be executed.\n";
?>

The output of the above code will be:

Exception: Uncaught exception occurred!

PHP Errors and Logging Constants

The constants below are always available as part of the PHP core.

The above values (either numerical or symbolic) are used to build up a bitmask that specifies which errors to report. The bitwise operators can be used to combine these values or mask out certain types of errors.

Complete PHP Error Handling Reference

For a complete reference of all PHP Error Handling functions, see the complete PHP Error Handling Reference.