The PHP set_error_handler() function sets a user-defined error handler function.

<?php
// user-defined error handler function
function my_error_handler($errno, $errstr, $errfile, $errline) {
    echo '<strong>Error:</strong> [' . $errno . ']' . $errstr . '<br>';
    echo ' Error on line ' . $errline . ' in ' . $errfile . '<br>';
}

// set user-defined error handler function
set_error_handler('my_error_handler');

$test = 2;

// Trigger error
if ($test > 1) {
    trigger_error(' A custom error has been triggered');
}
?>

Error: [1024] A custom error has been triggered
Error on line 19 in /home/test.php

set_error_handler(errorhandler, E_ALL | E_STRICT)