This is a tool to allow a programmer to easily insert a new associative index into a desired position in a PHP associative array.

associative_array_splicer.php

<?php
/**
 * Insert an associative array into a specific position in an array.
 *
 * @param array $original
 *   The original array to add to.
 * @param array $new
 *   The new array of values to insert into the original.
 * @param int $offset
 *   The position in the array ( 0 index ) where the new array should go.
 *
 * @return array
 *   The new combined array.
 */
function array_splice_assoc(array $original, array $new, int $offset):array {
  return array_slice($original, 0, $offset, TRUE)
    + $new
    + array_slice($original, $offset, NULL, TRUE);
}

/**
 * Insert an associative array into a specific position in an array.
 *
 * @param array $original
 *   The original array to add to.
 * @param string $key
 *   The array key to insert as.
 * @param array $new
 *   The new array of values to insert into the original.
 * @param int $offset
 *   The position in the array ( 0 index ) where the new array should go.
 *
 * @return array
 *   The new combined array.
 */
function array_splice_assoc_key(array $original, string $key, array $new, int $offset):array {
  $new = [$key => $new];

  return array_slice($original, 0, $offset, TRUE)
    + $new
    + array_slice($original, $offset, NULL, TRUE);
}


/**
 * Example Usage...
 */
$external_array = [
  'First' => ['something', 'another thing', 'tertiary thing'],
  'Second Association' => [
    '#type' => 'text',
    '#title' => 'My really cool form array element',
    '#value' => 435,
  ],
  'Third Position' => [
    'Person' => [
      'name' => 'Bobby',
      'phone' => '321-648-8888',
    ],
  ],
  'Fourth thing' => 'Rando Value',
  '5th Element' => [1, 2, 3, 5, 'I declare a thumb war'],
];

/*
 * I want to insert a new associative index between 'Second Association' and
 * 'Third Position'. There is no built-in PHP function to do this in one simple
 * step. Requiring extra work. This is a shortcut.
 *
 * The array I want to insert is below.
 */
$my_new_array = [
  'Mutants' => ['Wolverine', 'Jean Grey', 'The Beast'],
  'Turtles' => ['Donatello', 'Raphael', 'Michelangelo', 'Leonardo'],
];

// You can either create the array with key yourself
$new_arr = [
  'my_new_array' => $my_new_array
];

// Then insert
$result = array_splice_assoc($external_array, $new_arr, 2);

// Or skip the extra wrapping step above
$result2 = array_splice_assoc_key($external_array, 'my_new_array', $my_new_array, 2);

/**
 * The result will look like this:
 *
 * $result = [
 *   'First' => ['something', 'another thing', 'tertiary thing'],
 *   'Second Association' => [
 *     '#type' => 'text',
 *     '#title' => 'My really cool form array element',
 *     '#value' => 435,
 *   ],
 *   'my_new_array' => [
 *     'Mutants' => ['Wolverine', 'Jean Grey', 'The Beast'],
 *     'Turtles' => ['Donatello', 'Raphael', 'Michelangelo', 'Leonardo'],
 *   ],
 *   'Third Position' => [
 *     'Person' => [
 *       'name' => 'Bobby',
 *       'phone' => '321-648-8888',
 *     ],
 *   ],
 *   'Fourth thing' => 'Rando Value',
 *   '5th Element' => [1, 2, 3, 5, 'I declare a thumb war'],
 * ];
 *
 * This is really handy in Drupal form arrays and other arrays where positioning
 * is important to the output.
 */