Basic PHP/HTML code question

Hi, I have been trying to write or find a pre-written script of a combo-box which would allow one to select a category from one drop-down list, then be given related options within a secondary list before clicking a submit button. Is there anyone who knows where I can find this or an easy way to accomplish this in PHP? Sorry if this is a basic question but I have only begun learning coding PHP so some items are foreign at this point. Thanks for your understanding and any assistance. C.