php code for comment system

$(document).ready(function(){
	// When user clicks on submit comment to add comment under post
	$(document).on('click', '#submit_comment', function(e) {
		e.preventDefault();
		var comment_text = $('#comment_text').val();
		var url = $('#comment_form').attr('action');
		// Stop executing if not value is entered
		if (comment_text === "" ) return;
		$.ajax({
			url: url,
			type: "POST",
			data: {
				comment_text: comment_text,
				comment_posted: 1
			},
			success: function(data){
				var response = JSON.parse(data);
				if (data === "error") {
					alert('There was an error adding comment. Please try again');
				} else {
					$('#comments-wrapper').prepend(response.comment)
					$('#comments_count').text(response.comments_count); 
					$('#comment_text').val('');
				}
			}
		});
	});
	// When user clicks on submit reply to add reply under comment
	$(document).on('click', '.reply-btn', function(e){
		e.preventDefault();
		// Get the comment id from the reply button's data-id attribute
		var comment_id = $(this).data('id');
		// show/hide the appropriate reply form (from the reply-btn (this), go to the parent element (comment-details)
		// and then its siblings which is a form element with id comment_reply_form_ + comment_id)
		$(this).parent().siblings('form#comment_reply_form_' + comment_id).toggle(500);
		$(document).on('click', '.submit-reply', function(e){
			e.preventDefault();
			// elements
			var reply_textarea = $(this).siblings('textarea'); // reply textarea element
			var reply_text = $(this).siblings('textarea').val();
			var url = $(this).parent().attr('action');
			$.ajax({
				url: url,
				type: "POST",
				data: {
					comment_id: comment_id,
					reply_text: reply_text,
					reply_posted: 1
				},
				success: function(data){
					if (data === "error") {
						alert('There was an error adding reply. Please try again');
					} else {
						$('.replies_wrapper_' + comment_id).append(data);
						reply_textarea.val('');
					}
				}
			});
		});
	});
});

//...
// If the user clicked submit on comment form...
if (isset($_POST['comment_posted'])) {
	global $db;
	// grab the comment that was submitted through Ajax call
	$comment_text = $_POST['comment_text'];
	// insert comment into database
	$sql = "INSERT INTO comments (post_id, user_id, body, created_at, updated_at) VALUES (1, " . $user_id . ", '$comment_text', now(), null)";
	$result = mysqli_query($db, $sql);
	// Query same comment from database to send back to be displayed
	$inserted_id = $db->insert_id;
	$res = mysqli_query($db, "SELECT * FROM comments WHERE id=$inserted_id");
	$inserted_comment = mysqli_fetch_assoc($res);
	// if insert was successful, get that same comment from the database and return it
	if ($result) {
		$comment = "<div class='comment clearfix'>
					<img src='profile.png' alt='' class='profile_pic'>
					<div class='comment-details'>
						<span class='comment-name'>" . getUsernameById($inserted_comment['user_id']) . "</span>
						<span class='comment-date'>" . date('F j, Y ', strtotime($inserted_comment['created_at'])) . "</span>
						<p>" . $inserted_comment['body'] . "</p>
						<a class='reply-btn' href='#' data-id='" . $inserted_comment['id'] . "'>reply</a>
					</div>
					<!-- reply form -->
					<form action='post_details.php' class='reply_form clearfix' id='comment_reply_form_" . $inserted_comment['id'] . "' data-id='" . $inserted_comment['id'] . "'>
						<textarea class='form-control' name='reply_text' id='reply_text' cols='30' rows='2'></textarea>
						<button class='btn btn-primary btn-xs pull-right submit-reply'>Submit reply</button>
					</form>
				</div>";
		$comment_info = array(
			'comment' => $comment,
			'comments_count' => getCommentsCountByPostId(1)
		);
		echo json_encode($comment_info);
		exit();
	} else {
		echo "error";
		exit();
	}
}
// If the user clicked submit on reply form...
if (isset($_POST['reply_posted'])) {
	global $db;
	// grab the reply that was submitted through Ajax call
	$reply_text = $_POST['reply_text']; 
	$comment_id = $_POST['comment_id']; 
	// insert reply into database
	$sql = "INSERT INTO replies (user_id, comment_id, body, created_at, updated_at) VALUES (" . $user_id . ", $comment_id, '$reply_text', now(), null)";
	$result = mysqli_query($db, $sql);
	$inserted_id = $db->insert_id;
	$res = mysqli_query($db, "SELECT * FROM replies WHERE id=$inserted_id");
	$inserted_reply = mysqli_fetch_assoc($res);
	// if insert was successful, get that same reply from the database and return it
	if ($result) {
		$reply = "<div class='comment reply clearfix'>
					<img src='profile.png' alt='' class='profile_pic'>
					<div class='comment-details'>
						<span class='comment-name'>" . getUsernameById($inserted_reply['user_id']) . "</span>
						<span class='comment-date'>" . date('F j, Y ', strtotime($inserted_reply['created_at'])) . "</span>
						<p>" . $inserted_reply['body'] . "</p>
						<a class='reply-btn' href='#'>reply</a>
					</div>
				</div>";
		echo $reply;
		exit();
	} else {
		echo "error";
		exit();
	}
}