$str = 'abcdefgh';
$arr = str_split($str,2);

array(4) {
 [0]=>
 string(2) "ab"
 [1]=>
 string(2) "cd"
 [2]=>
 string(2) "ef"
 [3]=>
 string(2) "gh"
}

$str = 'abcdefgh';
$arr = str_split($str);
$i = 0;
$limit = 3;
$num = count($arr);
while($i <= $num-1){
  $temp = array();
  $for_countbu = ($num-$i) >= $limit ? $limit : $num - $i;
  for($j = 0; $j < $for_countbu; ++$j)
  {
    $temp[] = $arr[$i];
    ++$i;
  }
  $one = implode('',$temp);
  $result[] = $one;
}
print_r($result);

array(4) {
 [0]=>
 string(2) "ab"
 [1]=>
 string(2) "cd"
 [2]=>
 string(2) "ef"
 [3]=>
 string(2) "gh"
}