How to generate passoword in php with specific length?

function generate_password( $length = 8 ) {
  $chars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
  $password = substr( str_shuffle( $chars ), 0, $length );
  return $password;
 }