PHP – I want the user to have no possibility to upvote more than one time

I am working on a small social network with PHP and MySQL I want the users to have the possibility to upvote a post, and I already did this but the problem is that I want the user to be able to upvote only one time.

Here is my PHP code:

if (isset($_POST['heart']))
{ 
  $identificateur = $_POST['hide'];
  $Q = "UPDATE posts  SET avis = avis + 1 where id=$identificateur ";
  $bdd->query($Q);
}

This is the form of the upvote button :

<form action="p.php" method="POST">
        <label><?php 
        if ($avis != 0)
                    {
                        echo $avis; 
                    }
        ?></label>
        <input type="hidden" value="<?php echo "$id" ?>" name="hide">
        <input type="submit" value=" " id="heart" name="heart"> 
</form>

Every user should have the right to click on the up vote icon only once. Thank you very much in advance.

Answer

You need to have a second table, that has two fields, post_id and voter_id. (post_voter).post_id has a foreign key containing the id of the post table and voter_id has foreign key to the user table. This table specifies voters of a post and posts that users have voted for. When a user tries to upvote a post, you must check post_voter table and get count of rows which have the post_id equal to the post’s id and voter_id equal to the user’s id. If count = 0, this means the user has not upvoted the post, yet, and he can vote for it now. Otherwise, the user upvoted the post already and he can’t upvote it again.