Description:
------------
see test script, there is some zval/GC data structure, that is currently not shrunk

this is an issue for workers that normally process small and large requests occasionally, as once the data structure inside php engine is enlarged, it never goes back and memory per worker/process stays allocated

Test script:
---------------
function createGraph($c) {
    $objs = [];
    for ($i = 0; $i < $c; $i++) {
        $obj = new \stdClass();
        $objs[] = $obj;
        $obj->x = $objs[mt_rand(0, count($objs) - 1)];
    }
}

for ($i = 0; $i < 5; $i++) {
    createGraph(2 * 1000 * 1000);
    
    gc_collect_cycles();
    echo round(memory_get_usage() / (1024 * 1024), 3) . " MiB\n";
}

Expected result:
----------------
memory usage stays below 1 MiB

Actual result:
--------------
16.361 MiB
16.361 MiB
16.361 MiB
16.361 MiB
16.361 MiB