array_insert()

array_insert.php

<?php

/**
 * Insert an array into another array before/after a certain key
 *
 * @param array $array The initial array
 * @param array $pairs The array to insert
 * @param string $key The certain key
 * @param string $position Wether to insert the array before or after the key
 * @return array
 */
function array_insert( $array, $pairs, $key, $position = 'after' ) {
	$key_pos = array_search( $key, array_keys( $array ) );

	if ( 'after' == $position )
		$key_pos++;

	if ( false !== $key_pos ) {
		$result = array_slice( $array, 0, $key_pos, true );
		$result = $result + $pairs;
		$result = $result + array_slice( $array, $key_pos, null, true );
	}
	else {
		$result = $array + $pairs;
	}

	return $result;
}

After some minor modification, the original function by scribu now maintains array keys, allowing it to work with associative arrays.