Display ad_id (or anything really) according to category ID. Thank you @garyj

ad_code.php

<?php 

global $post;

$lookup = array(
	// category ID => ad ID
	'1084' => '600',
	'1085' => '601',
	'1095' => '602',
	'1086' => '603',
	'1088' => '605',
	'1089' => '606',
	'1090' => '607',
	'1091' => '608',
	'1092' => '609',
	'1093' => '610',
	'1094' => '611'
	// ...
);

// Get all categories that the post is assigned to
$categories = get_the_category($post->ID);

// Loop through
foreach ($categories as $category) {
	// If category is in the lookup array...
	if (in_array($category->cat_ID, array_keys($lookup))) {
		// ...grab the corresponding ad id, then kill the loop
		$sf_product_id = $lookup[$category->cat_ID];
		break;
	}
}

// If no ad ID, set a default.
$default_ad_id = 604;
$sf_product_id = isset( $sf_product_id ) ? $sf_product_id : $default_ad_id;