mysql question

I know this isn't the mysql area... but many of you seem very knowledgeable in this area and I would appreciate your input... What kind of field setup should I create if I want a field which will, essentially hold several distinct items, but never a set number of them? I am going to be selecting these items (or the relational pointers to them) from a multi-select list (ie. this image belongs in the following categories). After that, I will need to output the contents of this field to an array. That part I know how to do using php as long as it's a big string, but I am guessing there is a better way to do this. From the manual, it seems that a SET field would work, but I'd need the SET to be the contents of a field from another table. For example, I have an image with an accession number of 1031-LH0152. It will fit in the categories of "Military Posters", "McLaughlin - Colonel Robert Samuel", and "Ontario Regiment". How do I get one field to hold all of the categories, and then what is the most efficient way to then transport that into an array with php? Thank you for the more-elaborate-than-just-php assistance. Marc