Graph in PHP

<?php

class Graph {

   private $adj;
   private $V;

   public function __construct($vertices) {
       $this->V = $vertices;
       $this->adj = array();
       for($i = 0; $i < $vertices; $i++) {
           array_push($this->adj, array());
       }
   }

   public function addEdge($from, $to) {
       array_push($this->adj[$from], $to);
       // If using an undirected Graph, uncomment this line:
       //array_push($this->adj[$to], $from);
   }

   public function size() {
       return $this->V;
   }

   public function getEdges($vertex) {
       if($vertex > $this->V) return null;
       return $this->adj[$vertex];
   }
}