exploding strings into arrays

[mattennant]

hello
can anyone point me in the right direction. my project involves a user uploading an image, and then using checkboxes choosing which categories the image should reside in. The output of the checkboxes are processed with the following code.

//check computer
if (isset($_POST['computer'])){
$com = NULL;
foreach ($_POST['computer'] as $key => $value){//loop through each
$com .="$value, ";
}
$com = substr($com, 0, -2);//cut off last comma and spce
//$com = TRUE;
} else {
$com = ' ';
}

this is then added to the category column of my database.

What i'm struggling with is when the user wants to update that information. I would like the page to apear with the image and the checkboxes previously selected checked. I'm drawing a blank. Reading around i think it might involve using explode but really not sure, can anyone point me in the right direction.