PHP MYSQL edit form, trying to implode/explode row to checkboxes?

I can not find a way to "EXPLODE" the values into checkboxes on the edit form

This line is so close to the answer that I'm seriously wondering if this is a joke post.

Given your database structure as it is, you can use the explode() function to break the value into an array based on a delimiter. For example:

$values = "1,2,3,4";
$array_of_values = explode(",", $values);

After this, $array_of_values will contain an array of four elements with the values of 1, 2, 3, and 4 respectively.

When deciding whether or not to display the checkbox as checked, you can use the in_array() function to make that call:

if (in_array("2", $array_of_values)) {
  echo 'checked="checked"';
}

Beyond that, let me take this opportunity to second what @MarkB said in the comments on this question: it's a bad idea to hold multiple values in a single database field. It makes searching for things harder, it makes adding values harder... it just makes everything more complex than it needs to be.

The best way to handle this would be to have a many-to-many table which stores rows of items and categories, with one row for each category that an item belongs to.

In other words, where you currently have this:

Item | Category
---------------
123  | 1,2,3,4

You would instead have a table that looks like this:

Item | Category
---------------
123  | 1
123  | 2
123  | 3
123  | 4