Online PHP editor | refs for P23AV

<?php echo preg_replace_callback('~[A-Z]~', function($_) { return $_[0] + 32; }, 'Important: you are NOT allowed to use a built-in function that converts the string (or just one character) to lowercase (such as ToLower() in .NET, strtolower() in PHP , ...)! You\'re allowed to use all other built-in functions, however.');