I wrote a file to test the memory usage of PHP. I found that the memory occupied by variables after release is different between arrays and objects. I would like to ask why. The following is the code, running in PHP 7.0 CLI mode.

Array:

<?php
function convert($size){
    $unit=array('b','kb','mb','gb','tb','pb');
    echo @round($size/pow(1024,($i=floor(log($size,1024)))),2).$unit[$i]."\n";
}
function a(){
    $a = [];
    for($i=0;$i<1000000;$i++){
        $a[$i] = $i;
    }
    convert(memory_get_usage(true));
}
convert(memory_get_usage(true));
// 2M
a();
convert(memory_get_usage(true))
// 2M

Output:

2M
34M
2M

Object:

<?php
function convert($size){
    $unit=array('b','kb','mb','gb','tb','pb');
    echo @round($size/pow(1024,($i=floor(log($size,1024)))),2).$unit[$i]."\n";
}
function a(){
    $a = new stdClass;
    for($i=0;$i<1000000;$i++){
        $a->$i = $i;
    }
    convert(memory_get_usage(true));
}
convert(memory_get_usage(true));
a();
convert(memory_get_usage(true))

Output:

2M
68M
32M