class Lamp {
    //TRUE = on, FALSE = Off
    protected bool $currentState = FALSE; 
    
    public function turnOn() {
        $this->currentState = TRUE;
    }
    
    public function turnOff() {
        $this->currentState = FALSE;
    }
    
    public function getState() {
        return $this->currentState;
    }
    
    public function getStateString(): string {
        if ($this->currentState) {
            return 'On';
        } else {
            return 'Off';
        }
    }
}

class Button {
    protected Lamp $lamp;
    
    public function __construct(Lamp $l) {
        $this->lamp = $l;
    }
    
    public function On() {
        $this->lamp->turnOn();
    }
    
    public function Off() {
        $this->lamp->turnOff();
    }
}

$l = new Lamp();
$b = new Button($l);
echo $l->getStateString(); //Off

$b->On();
echo $l->getStateString(); //On

You can see one class depends one another. How we can fix this is with an example below.

interface DeviceInterface {
    public function turnOn();
    public function turnOff();
}

class Lamp implements DeviceInterface {
    //TRUE = on, FALSE = Off
    protected bool $currentState = FALSE; 
    
    public function turnOn() {
        $this->currentState = TRUE;
    }
    
    public function turnOff() {
        $this->currentState = FALSE;
    }
    
    public function getState() {
        return $this->currentState;
    }
    
    public function getStateString(): string {
        if ($this->currentState) {
            return 'On';
        } else {
            return 'Off';
        }
    }
}

interface ButtonInterface {
    public function On();
    public function Off();
}

class Button implements ButtonInterface {
    protected DeviceInterface $di;
    
    public function __construct(DeviceInterface $di) {
        $this->di = $di;
    }
    
    public function On() {
        $this->di->turnOn();
    }
    
    public function Off() {
        $this->di->turnOff();
    }
}

$l = new Lamp();
$b = new Button($l);
echo $l->getStateString(); //Off

$b->On();
echo $l->getStateString(); //On

What we done here. We create 2 interfaces and implement 2 other classes from them, so we can change Button class or Lamp class. but whatever we created instead of those will be work just like others . Why? Just because all of them derived from one interface. methods may do different jobs but they should have same properties and method names. Hope i can explain this.