Thank you for sharing your code. This is indeed how you would use the exchange’s PHP API to pull the minimum bid and maximum ask prices for a market. You can also have access to the latest price exchanged with $market_summary->last_price.
Once quick comment, you are checking for if ($ask == " ") {. This might work, but to be safe, I would write if ( ! $ask ) {. You are not interested in whether $ask is empty but whether it is truthy or falsy. (See http://php.net/manual/en/language.types.boolean.php)
kind regards