Show error message when clicked on the like button - php

Question

Hello I want to make a like system using PHP and MySQL when clicked on the like Button i also insert Data in the database but there is an error database value inserted but like value as 0 no increment and undefined error occurs . Can anybody help me solving this problem
There is my Like button code :
<?php
//// work with like box
$get_likes = mysqli_query($con,"SELECT * FROM `likes`");
if (mysqli_num_rows($get_likes)===1) {
$get = mysqli_fetch_assoc($get_likes);
// $uid = $get['uid'];
$total_likes = $get['total_likes'];
//echo $uid;
$total_likes = $total_likes + 1;
//echo $total_likes++;
}
if (isset($_POST['likebutton_'])) {
$like = mysqli_query($con,"UPDATE `likes` SET `total_likes` = '$total_likes'") or die(mysqli_error($con));
//$insert_Data = mysqli_query($con,"INSERT INTO `likes` (`uid`) VALUES('$username')") or die(mysqli_error($ocn));
header("Location:home.php");
}
else
{
echo "Error";
}
?>
this code work fine without insert Data
There is My liked with Data Insertd Code
<?php
////work with like box
$get_likes = mysqli_query($con,"SELECT * FROM `likes`");
if (mysqli_num_rows($get_likes)===1) {
$get = mysqli_fetch_assoc($get_likes);
// $uid = $get['uid'];
$total_likes = $get['total_likes'];
//echo $uid;
$total_likes = $total_likes + 1;
//echo $total_likes++;
}
if (isset($_POST['likebutton_'])) {
$like = mysqli_query($con,"UPDATE `likes` SET `total_likes` = '$total_likes'") or die(mysqli_error($con));
$insert_Data = mysqli_query($con,"INSERT INTO `likes` (`uid`) VALUES('$username')") or die(mysqli_error($ocn));
header("Location:home.php");
}
else
{
echo "Error";
}
?>
this is output i want to display my font-end page <?php echo $total_likes ;?> but it occur error
The error is Undefined Variable
I also try $total_likes="";
as global but still not work

Answer 1

Your code suffers from a race condition. What you should be doing is this pattern:
INSERT INTO likes (uid, total_likes) VALUES (?, 1)
ON DUPLICATE KEY SET total_likes=total_likes+1
Where you use bind_param to set the placeholder value to your UID.
Note that in your one query you set the total count of all likes to be +1. This is a huge mistake.