Convert string into slug with single-hyphen delimiters only

First strip unwanted characters

$new_string = preg_replace("/[^a-zA-Z0-9\s]/", "", $string);

Then changes spaces for unserscores

$url = preg_replace('/\s/', '-', $new_string);

Finally encode it ready for use

$new_url = urlencode($url);

function slug($z){
    $z = strtolower($z);
    $z = preg_replace('/[^a-z0-9 -]+/', '', $z);
    $z = str_replace(' ', '-', $z);
    return trim($z, '-');
}