[a,b,c,d] => [a => b, c => d]

chunk.php

<?php
  2
  3 $array = ['one', 'two', 'three', 'four', 'five', 'six'];
  4
  5 $chunks = array_chunk($array, 2);
  6 $firsts = array_map(function ($x) {return $x[0]; }, $chunks);
  7 $seconds = array_map(function ($x){return $x[1]; }, $chunks);
  8 $result = array_combine($firsts, $seconds);
  9 print_r($result);
 10