If you're going to use PHP, you're 99.9% likely going to be interacting with a database. So you also need to know SQL, along with how to use PHP to interact with DBMS's (e.g. the MySQLi, PostgreSQL, and PDO extensions, to name probably the most common).
To be competitive in today's market, you also need a good grasp of object-oriented programming principles and the specifics of using OOP in PHP. (I recommend Matt Zandstra's PHP 5 Objects, Patterns, and Practice.