I need a SQL query in a PHP file (prestashop)

General information for the website: We sell gold
Kind of development: Customization of existing website
Description of requirements/features: I'm trying to make a php file that will update prestashop prices every 30 minutes.
The process:
1) jQuery calls prices.php every 30 minutes, a cron job will do the same once a day ---- DONE
2) prices.php get the live value of gold and silver from a JSON file. This JSON file is in gold-feed.com (goldPrice, silverPrice)---- DONE
3) Prices.php: SQL query in prices,php select idProducts and 3 product feature fields from prestashop (typeMetal, weigth and bfactor) ---- NEEDS DOING
4) Prices.php: Update all the prices in prestashop BBDD. The formula is IF typemetal=gold then priceofIdProduct = goldPrice x weigth x bfactor ELSE IF typemetal=silver then priceofIdProduct = silverPrice x weigth x bfactor ---- PARTIALLY DONE

Notes:
price.php is in the prestashop base DIR (public_html/price.php)
IN SIMILAR URLS you can see a similar process. http://stackoverflow.com/questions/17151576/prestashop-product-features-php-sql

I attach the actual price.php and jQuery


setInterval(function(){
$('#divId').load('price.php');
}, 360000);

Extra services needed: Hosting
Similar urls: http://stackoverflow.com/questions/17151576/prestashop-product-features-php-sql
Extra notes: