Thank you oddz. I started looking for where in the script $music_genre was set and all of a sudden I remember that it was a part of another thought I had that I skipped later. So this line was probably something I worked on earlier and now got back to. Then I changed bits and pieces around it and I fugured out that I actually don't need that one.
Now I wrote the whole part again and it works without the error part. Sorry, it's been a while since I worked with this page and always hard to get back into the way I was thinking before.
So, now it's solved and the code looks like this:
<input type="checkbox" name="music_genre" id="music_genre<?php echo $row_rs_genres['cat_id']; ?>" value="<?php echo $row_rs_genres['cat_id']; ?>"/><?php echo $row_rs_genres['category']; ?></label>
And I have a loop around this that I made before that is working fine. I guess I was thinking about a loop or not when I was working with the page.
And thanx SpacePhoenix for the reminder about sanitizing. I have that in a lot of places in this script and I probably just forgot it here. I will try to search for it all over my code now and make sure it's a part of it.
And then I will probably start to change everything to MySqli... and that sounds really sad... after all the work...
And I'm also working in Dreamweaver that doesn't support mysqli... only because I'm always a beginner and play around with php. Then Dreamweaver is helping me out a lot... since it's the way I once learned this a couple of years ago... the wrong way I suppose.
And I'm 51 yo and not really happy to start learning the things all over again, the right way...