for($i = 0; $i <= 10_000; $i++) { 
    $ary[$i] = init_val($i);
}
[download]

means...

$ary[0] = init_val(0)
$ary[1] = init_val(1)
$ary[2] = init_val(2)
...
[download]

and...

$idx = 0;
for($i = 0; $i < 10_000; $i += 10) {
    $ary[$idx++] = init_val($i);
    $ary[$idx++] = init_val($i);
    $ary[$idx++] = init_val($i);
    ...
}
[download]

means...

$ary[0] = init_val(0)
$ary[1] = init_val(0)
$ary[2] = init_val(0)
...
$ary[10] = init_val(10)
$ary[11] = init_val(10)
$ary[12] = init_val(10)
...
[download]

It should be :

$idx = 0;
for($i = 0; $i < 10_000; $i += 10) {
    $ary[$idx++] = init_val($idx);
    $ary[$idx++] = init_val($idx);
    $ary[$idx++] = init_val($idx);
    ...
}
[download]

Am I wrong ?

BobiOne KenoBi ;)