array(8) {
  [0]=>  string(2) "a1"
  [1]=>  string(2) "b1"
  [2]=>  string(2) "a2"
  [3]=>  string(2) "b2"
  [4]=>  string(2) "a3"
  [5]=>  string(2) "b3"
  [7]=>  string(2) "b4"
  [9]=>  string(2) "b5"
}