<?php

$array = array('a', 'b', 'c');
$insert_array = array('one', 'two');
$key = 1;
$offset = 0;

$pos = isset($array[$key]) ? array_search($key, array_keys($array)) + $offset : 0;
$array = array_merge(array_slice($array, 0, $pos, TRUE), $insert_array, array_slice($array, $pos, NULL, TRUE));

var_dump($array);