This code allows to pass filename in the $file_name variable and function will return file extension only.

gistfile1.php

    <?php
/* Source: http://www.apphp.com/index.php?snippet=php-get-file-extension */
    function get_file_extension($file_name)
    {
        /* may contain multiple dots */
        $string_parts = explode('.', $file_name);
        $extension = $string_parts[count($string_parts) - 1];
        $extension = strtolower($extension);
        return $extension;
    }
    ?>