We are now going to implement the queue data structure using a PHP array. We have already seen that we can use the array_push() function to add an element at the end of the array. In order to remove the first element of the array, we can use the array_shift() function of PHP, and for the peek function, we can use the current() function of PHP. Here is how the code will look, based on our discussion:

class AgentQueue implements Queue {

    private $limit; 
    private $queue; 

    public function __construct(int $limit = 20) { 
      $this->limit = $limit; 
      $this->queue = []; 
    } 

    public function dequeue(): string { 

      if ($this->isEmpty()) { 
          throw new UnderflowException('Queue is empty'); 
      } else { 
          return array_shift($this->queue); 
      } 
    } 

    public function enqueue(string $newItem) { 

      if (count($this->queue) < $this->limit) { 
          array_push($this->queue, $newItem); 
      } else { 
          throw new OverflowException('Queue is full'); 
      } 
    } 

    public function peek(): string { 
      return current($this->queue); 
    } 

    public function isEmpty(): bool { 
      return empty($this->queue); 
    } 

}

Here, we are maintaining the same principle we did for the stack. We want to define a fixed-size queue, with checking of overflow and underflow. In order to run the queue implementation, we can consider using it as an agent queue for a call center application. Here is the code to utilize our queue operations:

try { 
    $agents = new AgentQueue(10); 
    $agents->enqueue("Fred"); 
    $agents->enqueue("John"); 
    $agents->enqueue("Keith"); 
    $agents->enqueue("Adiyan"); 
    $agents->enqueue("Mikhael"); 
    echo $agents->dequeue()."
"; 
    echo $agents->dequeue()."
"; 
    echo $agents->peek()."
"; 
} catch (Exception $e) { 
    echo $e->getMessage(); 
} 

This will produce the following output:

Fred
John
Keith