Is random_string() function generate unique string?

Use This code for random string.
public function genRandomString() {
$length = 20;
$characters = "O123456789ABCDEFGHIJKLMNPQRSTUVWXYZabcdefghijklmnpqrstuvwxyz~*^";
$string = '';
for ($p = 0; $p < $length; $p++) {
$string .= $characters[rand(0, strlen($characters))];
}
return $string;
}