An interface, as described further on the php.net website here, is basically a abstract class that defines a skeleton of what a class will have to at least implement, this allows for classes to implement a set skeleton and you will know what functions that class will have to implement.
The interface is just that, a skeleton of a base class. e.g. a interface could be a shape, and the classes that implement the interface called shape could be circle, square etc..because they will all have the basic functions for example, shape sides, colour etc.
Here is some php code, I always find looking over code easier to understand the basics of things.
printOutAWord("Printing out a word"); $newWorker->printOutAArray(array("Array","words","are","here")); ?>
And the output would be this
Printing out a word Array (  => Array  => words  => are  => here )