$.ajax post <<--- Why this easy code doesn't work????

$.ajax post <<--- Why this easy code doesn't work????

Why success alert is not popped up?

[post.php]
<!DOCTYPE html>
<html>
<head>
<script>
$(document).ready(function() {
$('#post_button').click(function() {
$text = $('#text_p').val();
$.ajax({
type: "POST",
cache: false,
url: "save.php",
data: "text="+$text+"room="+"room1",
success: function(data) {
alert(data);
}
});//j.ajax
});
});

</script>
 
</head>
<body>
sfefe
<div id ="stage"> Stage </div>
<form name="form1" action="" method="post">
<input type ="text" name="text_p" id ="text_p" />
<input type="submit" name ="post_button" id ="post_button"> click this post button! 
</form>
</body>
</html>


[save.php]
<?php
header("Content-Type: application/json", true);
if (isset($_POST['text'])&&isset($_POST['room'])) {
$name = trim($_POST['text']);
$room = trim($_POST['room']);
echo $room;
}
?>