An example of how creating cycles in variables leads to

Mem Test Output

Without manually calling gc_collect_cycles()
============================================

Fatal error: Allowed memory size of 67108864 bytes exhausted (tried to allocate 139264 bytes) in - on line 27


GC Statistics
-------------
Runs:               0
Collected:          0
Root buffer length: 254
Root buffer peak:   260

      Possible            Remove from  Marked
        Root    Buffered     buffer     grey
      --------  --------  -----------  ------
ZVAL       261       260          6         0
ZOBJ         0         0          0         0



Manually calling gc_collect_cycles()
====================================

Max memory that should be 'live' is 5120 kB 
used:     358688 | allocated:     524288 | peak:     786432


GC Statistics
-------------
Runs:               1000
Collected:          2000
Root buffer length: 0
Root buffer peak:   5

      Possible            Remove from  Marked
        Root    Buffered     buffer     grey
      --------  --------  -----------  ------
ZVAL      1007      1007          6      2002
ZOBJ         0         0          0         0

memTest.php

<?php

$performGC = false;

if ($performGC) {
    echo "GC collection will be done - app should not crash.\n";
}
else {
    echo "GC collection won't be done - app should crash.\n";
}
$dataSizeInKB = 128;

//Change this line if you tweak the parameters above.
ini_set('memory_limit', "64M");

for ($y=0 ; $y<$dataSizeInKB ; $y++) {
    for ($x=0 ; $x<32 ; $x++) { //1kB
        $memData .= md5(time() + (($y * 32) + $x));
    }
}

file_put_contents("memdata.txt", $memData);

// This function creates a cyclic variable loop
function useSomeMemory($x) {
    $data = [];
    $data[$x] = file_get_contents("memdata.txt");
    $data[$x + 1] = &$data;
};


for($x=0 ; $x<1000 ; $x++) {
    useSomeMemory($x);
    if ($performGC == true) {
        gc_collect_cycles();
    }
}

printf("\nused: %10d | allocated: %10d | peak: %10d\n",
       memory_get_usage(),
       memory_get_usage(true),
       memory_get_peak_usage(true)
);