PHP rand string

function randString(){
	$n = 20;
	$characters = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
	$string = "";  
	$l = strlen($characters);
	 
	for ($i = 0; $i < $n; $i++) {
        $string .= $characters[rand(0, $l)];
    }
	
	return $string;
}