test.php

<?php

function filename_safe($filename) {
        $temp = $filename;
        $temp = strtolower($temp);
        $temp = str_replace(" ", "_", $temp);
        // Loop through string
        $result = '';
        for ($i=0; $i<strlen($temp); $i++) {
            if (preg_match('([0-9]|[a-z]|_)', $temp[$i])) {
                $result = $result . $temp[$i];
            }
        }
        return $result;
    }

?>