PHP & Javascript are quite similar languages but using javascript can build some smart flowing scripts / sites...
They both have their strong points. PHP: serverside which means it's you can't disable it (harder to hack), but to run the script you must refresh the page causing longer load time. Javascript: clientside, quick without refresh, can be used with DOM and AJAX to make really powerful scripts. I use both, PHP for serverside validation and inserting data into databases and javascript in tandem with DOM and AJAX to make interactive quick loading scripts.
They both have their strong points, you just have to ask yourself which is more important, reliablity of funtionality, this would depend on your dermographic =]