You can reduce a two-dimensional array into one-dimensional using array_reduce and array_merge. (PHP>=5.3.0)
<?php
$two_dimensional = array();
$two_dimensional['foo'] = array(1, 2, 3);
$two_dimensional['bar'] = array(4, 5, 6);
$one_dimensional = array_reduce($two_dimensional, 'array_merge', array());
# becomes array(1, 2, 3, 4, 5, 6)