php 查看数组占用的内存大小

<?phpfunction convert($size){    $unit=array('b','kb','mb','gb','tb','pb');    return @round($size/pow(1024,($i=floor(log($size,1024)))),2).' '.$unit[$i];}        $start = memory_get_usage();          //$a = array_fill(0, 10000, 1);        $a = [];        $b = [];        for ($i=0; $i<10000;$i++){            $a[] = 'chenghuan';        }        $mid = memory_get_usage(true); //10k elements array;          echo 'argv:', convert($mid) , '<br>';          for ($j=0; $j<10000;$j++){            $a[] = 'chenghuan';        }        $end = memory_get_usage(true); //10k elements array;          echo 'argv:',  convert($end)  , '<br>';