Simple stack abstraction made in PHP

Stack.php

<?php
namespace Stack;

class Stack {

    private $stack;

    function __construct() {
        $this->initStack();
    }

    private function initStack() {
        $this->stack    = Array(); 
    }

    public function getTop() {
        if ($this->isEmpty)   return NULL;

        $size   = $this->getSize;

        return $this->stack[$size-1];
    }

    public function push($element) {
        $this->stack[]  = $element; 
    }
    
    public function pop() {
        $element        = array_pop($this->stack);
        
        return $element;
    }

    public function getSize() {
        $size   = count($this->stack);

        return $size;
    }

    public function isEmpty() {
        if (empty($this->stack))    return TRUE;

        return FALSE;
    }
    
}

?>