Split array into 4-element chunks then implode into strings

$array = [1, 2, 3, 4, 5, 6, 7, 8, 9];
$chunk = array_chunk($array, 4);
var_dump(array_map(fn($item) => implode(', ', $item), $chunk));

array(3) {
  [0]=> string(7) "1, 2, 3, 4"
  [1]=> string(7) "5, 6, 7, 8"
  [2]=> string(1) "9"
}

$array = [1, 2, 3, 4, 5, 6, 7, 8, 9];

foreach ($array as $i => $v) {
    $group = (int) ($i / 4);
    if (isset($result[$group])) {
        $result[$group] .= ", $v";
    } else {
        $result[$group] = "$v";
    }
}
var_export($result);

var_export(
    array_map(
        fn($v) => implode(', ', $v),
        array_chunk($array, 4)
    )
);

$result = [];
while ($array) {
    $result[] = implode(', ', array_splice($array, 0, 4));
}
var_export($result);

array (
  0 => '1, 2, 3, 4',
  1 => '5, 6, 7, 8',
  2 => '9',
)