Zugesichertes und tatsächlich nutzbares memory_limit

<?php
$before = memory_get_usage();
$store = array();
while(true){
    //generate approx. 1MB
    for ($j = 0; $j<1024; $j++) {
        //generate approx. 1kB
        for ($i = 0; $i<10; $i++) {
            $store[] = 'abcdefghijklmnopqstuvwxy'; //4*25bytes assuming unicode
        }
    }
    $after = memory_get_usage();
    $script = ($after - $before)/1024/1024;
    $total = ($after)/1024/1024;
    echo  "script: $script, total: $total<br/>";
}
?>

script: 1.54736328125, total: 1.60482025146
script: 3.09492111206, total: 3.15237808228
script: 4.57940673828, total: 4.6368637085
script: 6.18883132935, total: 6.24628829956
script: 7.67325210571, total: 7.73070907593
script: 9.15767288208, total: 9.21512985229
script: 10.8920936584, total: 10.9495506287
script: 12.3765144348, total: 12.433971405
script: 13.8609352112, total: 13.9183921814
script: 15.3453559875, total: 15.4028129578
script: 16.8297767639, total: 16.8872337341
script: 18.3141975403, total: 18.3716545105
script: 20.2986183167, total: 20.3560752869
(...)
script: 100.4884758, total: 100.54593277
script: 101.972896576, total: 102.030353546
script: 103.457317352, total: 103.514774323
script: 104.941738129, total: 104.999195099
script: 106.426151276, total: 106.483608246
script: 107.910572052, total: 107.968029022
script: 109.394992828, total: 109.452449799
script: 110.879413605, total: 110.936870575
script: 112.363834381, total: 112.421291351
script: 113.848255157, total: 113.905712128
script: 115.332675934, total: 115.390132904
script: 116.81709671, total: 116.87455368
script: 118.301517487, total: 118.358974457
script: 119.785938263, total: 119.843395233
script: 121.270359039, total: 121.32781601
script: 122.754779816, total: 122.812236786
script: 124.239200592, total: 124.296657562
script: 125.723621368, total: 125.781078339
script: 127.208042145, total: 127.265499115

Fatal error: Allowed memory size of 134217728 bytes exhausted (tried to allocate 35 bytes) in /var/www/.../memusage.php on line 17