You are missing the point of using an interface. A class can have its own methods without implementing any interfaces. Apart from its own methods, if it also implements other interfaces, then it has to include and provide implementation details for all the methods specified in those interfaces. In this way, an interface provides a set of common methods that classes of a similar type are contracted to implement them. As an analogy, similar classes of building such as residence, office, classroom must have their own ways to turn on and off the light. To make sure that the builders of such buildings not to forget to implement these methods, they have to sign a contract (interface) which states that they must implement two methods called say turnOn() and turnOff() to turn on and off the light, however leaving out the details of how to do it.
More example PHP: Object Interfaces - Manual