quick implementation of SPL Priority Queue:

<?php

class PQtest extends SplPriorityQueue
{
    public function compare($priority1, $priority2)
    {
        if ($priority1 === $priority2) return 0;
        return $priority1 < $priority2 ? -1 : 1;
    }
}

$objPQ = new PQtest();

$objPQ->insert('A',3);
$objPQ->insert('B',6);
$objPQ->insert('C',1);
$objPQ->insert('D',2);

echo "COUNT->".$objPQ->count()."<BR>";

//mode of extraction
$objPQ->setExtractFlags(PQtest::EXTR_BOTH);

//Go to TOP
$objPQ->top();

while($objPQ->valid()){
    print_r($objPQ->current());
    echo "<BR>";
    $objPQ->next();
}

?>

output:

COUNT->4
Array ( [data] => B [priority] => 6 ) 
Array ( [data] => A [priority] => 3 ) 
Array ( [data] => D [priority] => 2 ) 
Array ( [data] => C [priority] => 1 )

I've used the SplPriorityQueue to determine an HTTP client's preferred MIME types.

<?php
$queue = new \SplPriorityQueue();
foreach (preg_split('#,\s*#', $_SERVER['HTTP_ACCEPT']) as $accept) {
    $split = preg_split('#;\s*q=#', $accept, 2);
    $queue->insert($split[0], isset($split[1]) ? (float)$split[1] : 1.0);
}
foreach ($queue as $mime) {
    echo $mime, PHP_EOL;
}
?>

My browser sends:
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8

And this script outputs:
text/html
application/xhtml+xml
application/xml
*/*

A better example:
Accept: text/html, application/xml,text/css;q=0.4,text/plain; q=0.9, application/json;q=0.8

And this script outputs:
text/html
application/xml
text/plain
application/json
text/css

For a heap-based priority queue to be at its most effective, the "priority" should be something that can take on a wide range of values (lengths, timestamps, populations). It optimises the tasks of searching the queue for the appropriate place to insert an item (and inserting it); and removing the first item in the list.

Items may potentially be inserted into the queue wherever two adjacent items have different priorities. The heap structure is an efficient way of indexing such insertion points when there are many of them distributed throughout the list.

If you have a sharply-limited enumeration of possible priority values, then there are very few insertion possible insertion points - one for each priority value. In that situation, one can make the insertion points explicit (and thus eliminate the need to maintain a heap indexing them) by implementing your priority queue as a list of simple queues from which you draw successive items from the highest-priority nonempty queue.

<?php

/**
 * Description of PriorityQueue
 *
 * (c) lsroudi http://lsroudi.com/ <[email protected]>
 * 
 * For the full copyright and license information, please view the LICENSE
 * file that was distributed with this source code.
 */
interface PriorityLoggerInterface {

    public function insert($value, $priority);
}

class PriorityLogger extends SplPriorityQueue implements PriorityLoggerInterface {
    
}

class Logger {

    const ERROR = 3;
    const NOTICE = 1;
    const WARNING = 2;

    private $priorityLogger;

    public function __construct(PriorityLoggerInterface $priorityLogger)
    {
        $this->priorityLogger = $priorityLogger;
    }

    public function addMessage($value, $priority)
    {
        $this->priorityLogger->insert($value, $priority);
    }

    public function getPriorityLogger()
    {
        return $this->priorityLogger;
    }

}

$priorityLogger = new PriorityLogger();

$logger = new Logger($priorityLogger);
$logger->addMessage('Message with notice type', Logger::NOTICE);
$logger->addMessage('Message with warning type', Logger::WARNING);
$logger->addMessage('Message with error type', Logger::ERROR);

$priorityLoggerQueue = $logger->getPriorityLogger();

foreach ($priorityLoggerQueue as $queue){
    print $queue . PHP_EOL;
}

//Résultat
//Message with error type
//Message with warning type
//Message with notice type
?>