<?php

$parent = [ 
  1 => Array ( 
        0 => 'test1',
        1 => 'test2'
  ),
  2 => Array ( 
        0 => 'test1_1',
        1 => 'test2_2'
  )
];

$result = [];
foreach (array_keys($parent) as $i => $k) {
    $result[$k] = array_column($parent, $i);
}


print_r($result);