"good enough" guid/uuid in PHP?

So once upon a time I used 

$key = time().".".getmypid();   

For a UID (unique ID) in PHP. Maybe I wanted things to sort a bit? But of course there's always plain old

 

$key = uniqid("",true);

(which says "" for prefix, and more entropy.)

if(preg_match('/[^\.a-z_\-0-9]/i', $id)){

    echo "bad id";

}

could be a check if you're using this for a filename, and want to make sure it's not too dangerous to try and open... (shenanigans changing the folder or escaping out)