<?php
$data = array(1=>'a', 2=>'b', 3=>'c', 4=>'d', 5=>'e');
$keys = array_keys($data);
array_splice($data, 2, 0, array_pop($data));
$data = array_combine($keys, $data);
var_dump($data);

array(5) {
  [1]=>
  string(1) "a"
  [2]=>
  string(1) "b"
  [3]=>
  string(1) "e"
  [4]=>
  string(1) "c"
  [5]=>
  string(1) "d"
}