The Interface Segregation Principle states that clients should only implement interfaces they actually use. They should not be forced to implement interfaces they do not use. As per the definition found on Wikipedia:
"many client-specific interfaces are better than one general-purpose interface"
What this means is that we should split large and fat interfaces into several small and lighter ones, segregating it so that smaller interfaces are based on groups of methods, each serving one specific functionality.
Let's take a look at the following leaky abstraction that violates the ISP:
interface Appliance {
public function powerOn();
public function powerOff();
public function bake();
public function mix();
public function wash();
}
class Oven implements Appliance {
public function powerOn() { /* Implement ... */ }
public function powerOff() { /* Implement ... */ }
public function bake() { /* Implement... */ }
public function...