<?php

function addSubCategories( $props = array() )
{
	$props = array_merge_recursive(array(
		'parentCategory' => 'defaultParent',
		'subCategories' => array(
			'defaultName'
		)
	), $props);

	print_r($props);
}

addSubCategories(array(
	'parentCategory' => 'Technology',
	'subCategories' => array(
		'iPods'
	)
));

Array
(
    [parentCategory] => Array
        (
            [0] => defaultParent
            [1] => Technology
        )

    [subCategories] => Array
        (
            [0] => defaultName
            [1] => iPods
        )

)