- array_combine: documentation ( source)
- strtr: documentation ( source)
- range: documentation ( source)
<?php
echo strtr('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.', array_combine(range('A', 'Z'), range('a', 'z')));