PHP array management has some limits to merging associative arrays. These methods allow for creative positional merging of associative arrays by key. Think of this as array_splice for associative arrays. Another clever option I found recently is http://stackoverflow.com/a/13289272/1542064

array_splice_key.php

<?php

/**
 * Insert another array into an associative array after the supplied key
 *
 * @param string $key
 *   The key of the array you want to pivot around
 * @param array $source_array
 *   The 'original' source array
 * @param array $insert_array
 *   The 'new' associative array to merge in by the key
 *
 * @return array $modified_array
 *   The resulting merged arrays
 */
function array_splice_after_key( $key, $source_array, $insert_array ) { 
	return array_splice_key( $key, $source_array, $insert_array );
}

/**
 * Insert another array into an associative array before the supplied key
 *
 * @param string $key
 *   The key of the array you want to pivot around
 * @param array $source_array
 *   The 'original' source array
 * @param array $insert_array
 *   The 'new' associative array to merge in by the key
 *
 * @return array $modified_array
 *   The resulting merged arrays
 */
function array_splice_before_key( $key, $source_array, $insert_array ) { 
	return array_splice_key( $key, $source_array, $insert_array, -1 );
} 

/**
 * Insert another array into an associative array around a given key
 *
 * @param string $key
 *   The key of the array you want to pivot around
 * @param array $source_array
 *   The 'original' source array
 * @param array $insert_array
 *   The 'new' associative array to merge in by the key
 * @param int $direction
 *   Where to insert the new array relative to given $position by $key
 *   Allowed values: positive or negative numbers - default is 1 (insert after $key)
 *
 * @return array $modified_array
 *   The resulting merged arrays
 */
function array_splice_key( $key, $source_array, $insert_array, $direction = 1 ){
	$position = array_search( $key, array_keys( $source_array ) ) + $direction;

	// setup the return with the source array
	$modified_array = $source_array;

	if( count($source_array) < $position && $position != 0 ) {
		// push one or more elements onto the end of array
		array_push( $modified_array, $insert_array );
	} else if ( $position < 0 ){
		// prepend one or more elements to the beginning of an array
		array_unshift( $modified_array, $insert_array );
	} else {
		$modified_array = array_slice($source_array, 0, $position, true) +
            $insert_array +
            array_slice($source_array, $position, NULL, true);
	}
	return $modified_array;
}

@codearachnid I think for array_splice_before_key you would want direction to be 0, not -1 because you want the new element to appear in the "current" position of that key, and to bump that key to a higher position.

Great gist! I found 2 bugs in it while testing it out.

Line 61 should be:
$modified_array = $source_array + $insert_array;
and line 64 should be:
$modified_array = $insert_array + $source_array;

Otherwise the $insert_array is pre/appended as a single sub-array element instead of the array's elements being added individually.