Simple test to prove garbage collector does not detect circular reference if SPL object is used.

test.php

<?php

gc_enable();

class test
{
    public $spl;

    public function __construct()
    {
        $this->spl = new SplPriorityQueue();
        $this->spl->insert($this, 1);
    }
}

$i = 0;
while($i < 50000) {
    $test = new test();

    unset($test);
    gc_collect_cycles();

    echo sprintf('%1$04d: ', $i) . number_format(memory_get_usage()) . " B\n";
    $i++;
}