array_walk_recursive pass by reference

i have an array $parms = array('1' => 'a', 'b' => array(3 => 'test')); i want $mail = '1:a 3:test'; array_walk_recursive($parms, function($val, $key, &$mail) {     $mail .= ucwords($key) . ': '. ucwords($val) . "\n"; }, &$mail); The above function worked perfectly well but i am getting: Call-time pass-by-reference has been deprecated in /var/www/html/test.php on line 31 if i change it to array_walk_recursive($parms, function($val, $key, &$mail) { $mail .= ucwords($key) . ': '. ucwords($val) . "\n"; }, $mail); i am not getting the error but $mail is not being returned. ny solution ? Regards KK