(PHP 8 >= 8.5.0)
get_error_handler — Gets the user-defined error handler function
This function has no parameters.
   Returns the currently defined error handler (if any).
   If the built-in error handler is used null is returned.
  
The returned handler is the exact callable value that was passed to set_error_handler() to define it.
Example #1 get_error_handler() example
<?php
$handler = function (int $errno, string $errstr, ?string $errfile, ?int $errline) {
     echo "Error: " . $errstr . "\n";
};
var_dump(get_error_handler()); // NULL
set_error_handler($handler);
var_dump(get_error_handler() === $handler); // bool(true)
?>Prior to PHP 8.5.0, this functionality can be provided by the following polyfill:
<?php
if (!function_exists('get_error_handler')) {
    function noop_error_handler() {
    }
    function get_error_handler(): ?callable {
        $handler = set_error_handler('noop_error_handler');
        restore_error_handler();
        return $handler;
    }
}
?>