- print_r: documentation ( source)
- array_merge: documentation ( source)
<?php
$array1 = array ( 'productcode' => 218133, 'categoryid' => 315, 'color' => 'red' );
$array2 = array (10,220);
$array1['categoryid'] = array_merge((array)$array1['categoryid'], $array2);
print_r($array1);