PHP test array example...

array_insert.php

<?php

/**
 * Inserts values into an array after a given key.
 * 
 * Values from $insert are inserted after (or before) $key in $array. If $key
 * is not found, $insert is appended to $array using array_merge().
 * 
 * @param $array
 *   The array to insert into. Passed by reference and altered in place.
 * @param $key
 *   The key of $array to insert after.
 * @param $insert
 *   An array whose values should be inserted.
 * @param $before
 *   If TRUE, insert before the given key, rather than after it.
 *   Defaults to inserting after.
 * @author: Shawn M.
 * @link: http://marc.info/?l=php-general&m=133044704602038&w=2
 */

function array_insert(&$array, $key = '', $insert, $before = FALSE) {
	if (is_array($array)) { // Overkill?
		if (array_key_exists($key, $array)) {
			$i = 0;
			foreach($array as $k => $v) {
				if($k === $key) {
					$p = ($before) ? $i - 1 : $i + 1;
					break;
				}
				$i++;
			}
			$array = array_merge(array_slice($array, 0, $p), $insert, array_slice($array, $p, count($array) - $p));
		} else {
			$array = array_merge($array, $insert);
		}
	}
}

test.php

<?php

ini_set('display_errors', 1);
error_reporting(E_ALL);

header('Content-Type:text/plain');

$o_insert['new'] = array(
	'mickey' => 'mouse',
	'minnie' => 'mouse',
	'donald' => 'ducks',
	'daffy' => 'duck',
);

$o = array(
	
	'animals' => array(
		
		'dogs' => 'bark',
		'cats' => array (
			'sound1' => 'pur',
			'sound2' => 'meow',
		),
		
	),
	
	'misc' => array(
		
		'baz' => array(
			'fun' => 'times',
			'good' => 'days',
			'almost' => 'over',
		),
		
		'foo' => array(
			'what' => 'tf',
			'this' => 'is it',
			'bah' => 'humbug',
		),
		
		# 
		# 
		# -----> I want to insert $o_insert here! <-----
		# 
		# 
		
		'bar' => array(
			'who' => array(
				'deep' => array(
					'here' => 'goodbye',
				),
			),
		),
		
	),
	
	'materials' => array(
		
		'car' => 'mazda',
		'house' => array(
			'stories' => 2,
			'beds' => 3,
			'baths' => 2,
		),
		
	),
	
	'work' => array(
		
		'company' => 'Acme Inc.',
		'management' => array(
			'boss' => 'billy',
			'ceo' => 'suzy',
			'cfo' => 'betty',
		),
		
	),
	
);


# I've tried both:
# 
# http://eosrei.net/articles/2011/11/php-arrayinsertafter-arrayinsertbefore
# http://drupal.org/node/66183
# 

/**
* Inserts values into an array after a given key.
*
* Values from $insert_array are inserted after (or before) $key in $array. If
* $key is not found, $insert_array is appended to $array using array_merge().
*
* @param $array
*	The array to insert into. Passed by reference and altered in place.
* @param $key
*	The key of $array to insert after
* @param $insert_array
*	An array whose values should be inserted.
* @param $before
*	If TRUE, insert before the given key, rather than after it.
*	Defaults to inserting after.
*/
function array_insert(&$array, $key, $insert_array, $before = FALSE) {
	$done = FALSE;
	foreach ($array as $array_key => $array_val) {
		if (!$before) {
			$new_array[$array_key] = $array_val;
		}
		if ($array_key == $key && !$done) {
			foreach ($insert_array as $insert_array_key => $insert_array_val) {
				$new_array[$insert_array_key] = $insert_array_val;
			}
			$done = TRUE;
		}
		if ($before) {
			$new_array[$array_key] = $array_val;
		}
	}
	if (!$done) {
		$new_array = array_merge($array, $insert_array);
	}
	// Put the new array in the place of the original.
	$array = $new_array;
}

$o_key = $o['misc'];
array_insert($o_key, 'foo', $o_insert);
$o['misc'] = $o_key;

print_r($o);