PHP & MySQL function delete unchecked categories from database problem

I have a PHP function that allows users to pick which categories there post should be displayed in. I want to be able to delete a users picked category from the database when they unchecked a check box. But everything I tried seemed to delete all the users categories or not work at all. Can someone help me wit开发者_高级运维h this problem?

Here is part of my PHP function that should delete unchecked categories.

for ($x = 0; $x < count($query_cat_id); $x++){
    if($query_cat_id[$x] == $cat['id']){
        echo 'checked="checked"';
    } else if(!isset($query_cat_id[$x])) {
        $mysqli = mysqli_connect("localhost", "root", "", "sitename");
        $delete_id = mysqli_query($mysqli,"DELETE FROM posts_categories WHERE post_id = '" . $post_id . "'");
        if (!$delete_id) {
            print mysqli_error($mysqli);
        }
    }
}

This query is deleting all categories from your post because you are telling it to with you where clause. You are essentially saying, remove every record from the table posts_categories that has a post id = $post_id. You need to add to your where clause to make it more specific.

$delete_id = mysqli_query($mysqli,"DELETE FROM posts_categories WHERE post_id = '" . $post_id . "' AND categoryID = '". $query_cat_id[$x] ."'");

This now says the same thing you did but adds an extra qualifier. Only delete rows from the table where the post_id is $post_id and the ID of the category is (value of your unchecked check box).

Disclaimer: I am taking a guess at where you are storing your category IDs in your function($query_cat_id[$x]).