Type hinting means to define which class object we want to receive exactly as a parameter in a constructor

class BLock { }
class Lock {
    private $isLocked;
    
    public function __construct() {
        
    }
    
    public function lock() {
        $this->isLocked = true;
    }
    
    public function unLock() {
        $this->isLocked = false;
    }
    
    public function isLocked() {
        return $this->isLocked;
    }
}
class Chest {
    private $lock;
    
    public function __construct(Lock $lock) {
        $this->lock = $lock;
    }
    
    public function close() {
        $this->lock->lock();
        echo 'Closed' . PHP_EOL;
    }
    
    public function open() {
        if ($this->lock->isLocked()) {
            $this->lock->unLock();
        }
        echo 'Opened' . PHP_EOL;
    }
    
    public function isClosed() {
        return $this->lock->isLocked();
    }
}

$chest = new Chest(new Lock);
// $chest = new Chest(new Block);

$chest->open();
$chest->close();

In this example

Chest

class constructor waits to receive

Lock

class object. If we pass any other class object, we will get an error like the following: Fatal error:

Uncaught TypeError: Argument 1 passed to Chest::__construct() must be an instance of Lock, instance of BLock given