Stack implementation in PHP without array_push() and array_pop()

class Stack
{
     const SIZE = 3;
     private $a =  array();
     private $tos ; // Top of Stack

     public function Stack($topOfStack){  // Constructor
          $this->tos = $topOfStack;       
     }

     public function push($value){
          if(!self::isfull()){
               echo '<br>Pushed to stack pos : ' .$this->tos;
               $this->a[$this->tos++] = $value;                 
          } else {
               echo "<br>Stack overflow error (Stack size is " . self::SIZE . ")! Possible Data Loss !";
          }          
     }

     public function pop(){
          if(!self::isempty()) {
               echo '<br>Popped from stack pos : ' . ($this->tos - 1);
               return($this->a[--$this->tos]);               
          } else {
               echo "<br>Stack is empty! What to pop...!";
          }
     }
     
     private function isempty() {
          return ($this->tos == 0 ? 1 : 0 );
     }

     private function isfull() {
          return ($this->tos == self::SIZE ? 1 : 0 );            
     }
};


$s = new Stack(0);
$s->push(100);
$s->push(200);
$s->push(300);
$s->push(400);
$s->pop();
$s->pop();
$s->pop();
$s->pop();