Simple priority queue implementation in PHP

PnQueue.php

<?php
/**
 * A really simple priority queue implementation in PHP.
 * 
 * @author João Ferrão
 * @link https://github.com/jferrao
 */
class PnQueue
{
    
    /**
     * The array that will act as a queue
     * @var array
     */
    protected $queue = array();

    
    
    /**
     * Adds an item at the end of the queue or at the end of
     * a given priority queue if $priority is set.
     * @param mixed $item
     * @param int $priority
     * @throws Exception If $priority is not a numerical value
     */
    public function push($item, $priority = 0)
    {
        if (!is_numeric($priority)) {
            throw new Exception('Numeric priority value expected');
        }
        isset($this->queue[$priority]) || $this->queue[$priority] = array();
        array_push($this->queue[$priority], $item);
        ksort($this->queue);
        return $this;
    }
    
    /**
     * Returs the first element of the highest priority set, or
     * the first element of a given priority queue if $priority is set.
     * @param int $priority
     * @return mixed|null
     */
    public function pop($priority = null)
    {
        // Get the first message from the first priority queue
        if (null === $priority) {
            reset($this->queue);
            $priority = key($this->queue);
        }
        
        if (is_numeric($priority) && isset($this->queue[$priority])) {
            // Avoiding array_shift() since it requires a re-index process
            // on the array and it may be slower for large collections of items
            $item = array_pop(array_reverse($this->queue[$priority]));
            unset($this->queue[$priority][key($this->queue[$priority])]);
   
            // Remove priority queue if it's empty
            if (empty($this->queue[$priority])) {
                unset($this->queue[$priority]);
            }
            
            return $item;
        }
        return null;
    }
    
    /**
     * Purge the entire queue or only messages in a given
     * priority queue if $priority is set.
     * @param int $priority
     */
    public function purge($priority = null)
    {
        if (null === $priority) {
            $this->queue = array();
        } elseif (is_numeric($priority) && isset($this->queue[$priority])) {
            $this->queue[$priority] = array();
        }
    }
    
    /**
     * Returns the number of items in all priority queues or in
     * a single priority queue if $priority is set.
     * @param int $priority
     * @return int
     */
    public function count($priority = null)
    {
        switch ($priority) {
            case null:
                $count = 0;
                foreach ($this->queue as $priorityQueue) {
                    $count += count($priorityQueue);
                }
                return $count;
            case (is_numeric($priority) && isset($this->queue[$priority])):
                return count($this->queue[$priority]);
            default:
                return 0;
        }
    }
    
    /**
     * Dump the content of the entire queue or a single priority
     * queue if $priority is set.
     * @param int $priority
     * @return array|null
     */
    public function dump($priority = null)
    {
        if (null === $priority) {
            return $this->queue;
        } elseif (is_numeric($priority) && isset($this->queue[$priority])) {
            return $this->queue[$priority];
        }
        return null;
    }

}