如何将每个用户限制为每页一个喜欢

我有一个喜欢的系统，有点像Facebook。但是，用户可以尽可能多地喜欢页面。如何将其限制为每个用户（例如每个页面）一个？我拥有的代码是：

<?php
    session_start() ;
    $conn = mysqli_connect("","","","");
    $p_id = $_GET['post_id'];
    $result = mysqli_query($conn, "SELECT * FROM forum WHERE post_id = $p_id");
    $row = mysqli_fetch_assoc($result);
    mysqli_query($conn, "UPDATE forum SET likes=likes+1 WHERE post_id = '$p_id'") ;
    ?>
    <?php
    header("Location: forum.php?id=".$row['post_id']);
    die();
    ?>