for loop -- ununderstandable behavior - php

Question

I have have these for loops:
for ($x = 0; $x<2;$x++){
$allproducts = array();
for ($y = 0; $y<105;$y++) { //<<<<<<<<<<<<< this (105) is the number I mean
$allproducts[] = 'test'.$y;
}
echo "<pre>";
echo "allproducts 0 -- ";
var_dump($allproducts);
echo "</pre>";
for ($i = 0; $i < count($allproducts); $i++) {
$result = array_slice($allproducts, 0, 20);
echo "<pre>";
echo "result -- ";
var_dump($result);
echo "</pre>";
$allproducts = (array_diff($allproducts, $result));
echo "<pre>";
echo "allproducts 1 -- ";
var_dump($allproducts);
echo "</pre>";
}}
In real code (here:)
for ($x = 0; $x<1;){
$allproducts = array();
$abfrage = "SELECT sku FROM skulist";
if ($result = $sql->query($abfrage))
{
while ($row = $result->fetch_assoc()) {
$allproducts[] = $row['sku'];
}
}
for ($i = 0; $i < count($allproducts); ++$i) {
$result = array_slice($allproducts, 0, 20);
$allproducts = (array_diff($allproducts, $result));
//.... more code
}}
...I take data from DB instead of this for loop:
$allproducts = array();
for ($y = 0; $y<105;$y++) { //<<<<<<<<<<<<< this (105) is the number I mean
$allproducts[] = 'test'.$y;
}
but changed it to show/ for easier reconstruction.
The behavior is the same:
When I have 105 entries in my array, I don't get an empty array output for
echo "allproducts 1 -- ";
var_dump($allproducts);
which is the desired behavior.
When I change 105 to (eg.) 65, it outputs an empty arrays
allproducts 1 -- array(0)
at the end of the second nested loop:
for ($i = 0; $i < count($allproducts); $i++) {
Now I have three problems:
why does this happen? I don't get the difference
how can I achieve, that there is no empty array at the end? (stops my programs work)
I need 20 values in every $result array

Answer 1

for ($i = 0; $i < count($allproducts); $i++) {
// ...
$allproducts = (array_diff($allproducts, $result));
You are changing your $allproducts array while going through it! How can you expect it to work reliably then?
Instead, try:
foreach( $allproducts as $i => $product)
Although I must admit I'm not sure what you're trying to do.