PHP weird comparison - php

Question

I've got two inputs in registration form, one with login, one with password. After calling AJAX I'm trying to validate those data and I'm using function below:
function test_input($data) {
if( $data == $_POST["login"] && $data == "" ){
$GLOBALS["message"] .= "<p>Empty login.</p>";
return;
}
elseif( $data == $_POST["pwd"] && $data == "" ){
$GLOBALS["message"] .= "<p>Empty password.</p>";
return;
}
echo 'Good job.';
}
When I click submit button without writing anything I've got answer:
Empty login
Empty login
When I write something in one of them or both of them validation's ok. Why?
Edit - Missing code:
if ($_SERVER["REQUEST_METHOD"] == "POST") {
$login = test_input($_POST["login"]);
$pwd = test_input($_POST["pwd"]);
if($message!= ""){
echo $message;
}
else{
echo 'OK';
}
}

Answer 1

Perhaps refactor it something like:
function test_input($data, $field) {
if( $field == "login" && $data == "" ){
return "<p>Empty login.</p>";
}
elseif( $field == "pwd" && $data == "" ){
return "<p>Empty password.</p>";
}
echo 'Good job.';
}
and
$message = "";
if ($_SERVER["REQUEST_METHOD"] == "POST") {
$message .= test_input($_POST["login"], "login");
$message .= test_input($_POST["pwd"], "pwd");
if($message!= ""){
echo $message;
}
else{
echo 'OK';
}
}