DB design

Here's my problem... Each item in my database can belong to several of 50 categories. Right now I'm using a text field which contains a string of letters representing each category. (i.e. item A belongs to categories one and five so it's category field is " ONE FIV " To find data I use a select statement with a WHERE category LIKE "% FIV %" which gets everything that belongs to category 5.) What I want to know is there a better way to do this? It is possible that the number of categories will double in the near future. I'm using PHP 4.0.3 and MySQL 3.22.32 to create a dynamic website, right now it's getting may hits, but it is possible that could change as well. Thanks, -Tom