How do I send data via AJAX? - php

Question

I'm working on a social site. When I click the like button it runs the php page called "like.php", and inserts data into the data base BUT it is not receiving the post id, yet it will console log the post id from the jQuery.
here is my jQuery:
<script>
$(document).on('click', '.like_button', function() {
var postId = $(this).data('post-id');
console.log(postId);
$.ajax({
url: 'db/like.php',
type: 'POST',
data: { post_id: postId },
success: function(data) {
$('.like_count[data-post-id="' + postId + '"]').text(data);
console.log(data);
}
})
})
</script>
And here is my PHP:
<?php
session_start();
include("db.php");
$post_id = $_POST['post_id'];
$date = time();
// Get Current Post Likes
$sql = "select * from posts where id = '$post_id'";
$result = mysqli_query($conn, $sql);
$curr_likes = mysqli_fetch_assoc($result);
$new_likes = $curr_likes['likes'] + 1;
// Insert Into Likes table
$sql2 = "insert into likes (userid, postid, date) values ('$_SESSION[id]', '$post_id', '$date')";
mysqli_query($conn, $sql2);
if(isset($post_id)) {
echo "SET";
}else{
echo "NOT SET";
}
From the php page I'm getting "NOT SET"