PAID AND UNPAID HOSTING
Free PHP hosting
While free PHP hosting can be a simple/free solution for smaller personal websites this is in no way the right hosting solution for larger sites or any type of commercial sites. Almost all free PHP hosts place some kind of ads on your website, they are generally far less reliable then paid hosts and their customer support usually ... well, sucks
While using free hosts keep in mind that some might have script restrictions like disabled sending email or "fopen()" PHP function.
Update: because of the unreliable service or discontinued offering of free plans I decided I can no longer recommend free php web hosting companies that were listed here. To find free hosting please use Google search, but I would strongly recommend reading the text about paid php hosting below.
Paid PHP hosting
If you are serious about your website you should consider getting a paid host. Free hosts usually provide only limited scripting support (if any), they fill your pages with ads and (even worse) pop-ups, they can be very unreliable and you must wait a long time to get a reply (if at all) to your questions. Remember, you get what you pay for!
If you want your website to really succeed (especially if you plan a commercial site), paid hosting is a must. I want to show you that hosting with PHP enabled really isn't that expensive. I hand-picked a few php hosting companies with affordable plans and good customer support, see if any of them fits your needs.