If you don't know either and wanna start, learn PHP. If you already know one, just stick to that unless you need to learn the other. One bonus of PHP is its syntactical similarity to C, which is nice if you ever decide to do desktop app programming.
And php is very syntax forgiving. You don't spend hours deciding if you should write: include 'file.php'; include "file.php"; include('file.php'); include ('file.php'); include("file.php"); They all work just fine. And for the most part you don't need to worry about data type conversion, it's done for you magically, a number is a number when you need it to be or a string when you want to concatenate it or substring it, etc.