I cam across an interesting page this morning, on testing PHP knowledge, http://martinonphp.ghost.io/2014/01/27/evaluating-php-skill-level/
If you are a developer, then it is certainly worth taking a look at, making sure you know all constructs talked about. As the saying goes, ‘if all you have is a hammer, then everything looks like a nail to you.’ It is important to know more than one way of doing things and though you certainly do not need to know all PHP keywords/functions to write great code, it is always good to help you write higher quality code that is more simple to read while potentially being more efficient as well.
If you are hiring php developers, then this can be used as an inspiration. Some of the most common questions like telling the difference between get and post are not necessarily indicative of coding skill.