How do I add PHP code into a DB field

Hello, On my page, all my content is updated via a textarea online. If I include PHP code embeded with my HTML in the textarea, the database converts it to plain text, instead of code that can be used. It doesn't convert < to &lt; though, it's still in the database as <, just when it's read out again.. it don't work! I want to be able to call a random image script in PHP from the content that's stored in the DB. How do I do this? Thanks Tim