Php xhtml validator

1

Hey, Im trying to build a html validator, however im having trouble with figuring out where exactly to start. the purpose of this is that i have a cms that ive been writing and i have a section where someone can edit certain things in an html view. and i want to have a button they can click that will take the contents of that and check that it validates. anyone know of any good open source ones out there, or have any advice on the topic. Thanks in advance.