What is an interface in PHP, and what purpose does it serve in defining a contract for classes to follow?

An interface defines a set of method signatures that any class implementing that interface must provide, without specifying any actual implementation details itself, acting as a contract that guarantees any class implementing the interface will have those specific methods available, which lets different parts of your code depend on the interface rather than any particular concrete class, improving flexibility and testability.

interface Shape {
    public function area(): float;
}
class Circle implements Shape {
    public function __construct(private float $radius) {}
    public function area(): float { return M_PI * $this->radius ** 2; }
}