URL友好标题短划线不会转换为null

Question

i have the following code

$title = trim($tit);
$title = strtolower($tit);
$title = preg_replace("/\s/m", "_", $title);
$title = preg_replace("/\'/m", "_", $title);
$title = preg_replace("/[0-9+~!?@#;$%^&’*()\s]/", "", $title);

The problem arises when i have the following title

People are great - like or dislike?

And that converts to

people_are_great_�_like_or_dislike

I've tried many options like adding the dash in the final preg_replace in the code above. I tried to have a separate str_replace before the trim, after the preg_replaces etc. It doesn't seem to work.