Sort values in ascending groups representing every third value - php

Question

I have an array that I'd like to reorder by every 3rd item. So it's 1st, 4th, 7th and then 2nd, 5th, 8th and 3rd, 6th, 9th.
Input:
$items = ['1','2','3','4','5','6','7','8','9'];
Desired result:
['1','4','7','2','5','8','3','6','9']
Or it can be 3 separate arrays as well. Like this:
['1', '4', '7']
['2', '5', '8']
['3', '6', '9']
I tried array_chunk($items, count($items) / 3) but it just returns the same array divided into 3 equal arrays.
['1', '2', '3']
['4', '5', '6']
['7', '8', '9']
I don't know if I should use array_chunk and then something else to achieve what I want.
UPDATE: It does not have to be exactly 9 items in the input array. it can be shorter or longer, the key point is that it has to take every 3rd item as long as it finds any and so on...
So if there are 10 items:
$items = ['1','2','3','4','5','6','7','8','9','10'];
Desired result:
['1','4','7','10','2','5','8','3','6','9']

Answer 1

The simplest way I can think of is to loop over the list three times:
Start at element 0, skipping by 3, stop when past end of list
Start at element 1, skipping by 3, stop when past end of list
Start at element 2, skipping by 3, stop when past end of list
That can be easily achieved using C-style for loop syntax:
$maxKey = count($items) - 1;
$rearrangedItems = [];
for ( $key=0; $key<=$maxKey; $key+=3 ) {
$rearrangedItems[] = $items[$key];
}
for ( $key=1; $key<=$maxKey; $key+=3 ) {
$rearrangedItems[] = $items[$key];
}
for ( $key=2; $key<=$maxKey; $key+=3 ) {
$rearrangedItems[] = $items[$key];
}
If you needed to generalize to different offsets, you could put the whole thing into an outer loop rather than copying and pasting:
$offset = 3;
$maxKey = count($items) - 1;
$rearrangedItems = [];
for ( $start=0; $start<$offset; $start++ ) {
for ( $key=$start; $key<=$maxKey; $key+=$offset ) {
$rearrangedItems[] = $items[$key];
}
}

Answer 2

Try this
$items = array('1','2','3','4','5','6','7','8','9');
$tmp = [];
for ($i = 0; $i < count($items); $i++) {
$tmp[$i%3][] = $items[$i];
}
$result = array_merge($tmp[0], $tmp[1], $tmp[2]);
print_r($result);

Answer 3

Try this,
$items = array('1','2','3','4','5','6','7','8','9');
$tmp = [];
for ($i = 0; $i < count($items); $i++) {
$tmp[$i%3][] = $items[$i];
}
$result = call_user_func_array('array_merge', $tmp);
dd($result);
Output :
array('1','4','7','2','5','8','3','6','9')

Answer 4

I see this task as nothing more than a sorting task.
Sort each value by it modulus value when divided by 3. If the result is 0, then fallback to 3. Nothing extravagant, just simple, direct, logical, D.R.Y. programming.
Code: (Demo)
array_multisort(
array_map(fn($v) => $v % 3 ?: 3, $items),
$items
);

Answer 5

Pretty straightforward:
$items = array('1','2','3','4','5','6','7','8','9');
$items = array_chunk($items, 3); # Split into arrays of size 3
$items = array_map(null, ...$items); # Transpose the arrays
$items = array_merge(...$items); # Merge the arrays again
print_r($items);