USD to BTC conversion code
Your question is a bit confusing but it sounds like you are asking how to get a currency amount worth of bitcoin.    


  I want the user to be able to click the USD price and convert it to BTC instantly.    


The answer is simple, division. 

&lt;?php
$url = "https://blockchain.info/stats?format=json";
$stats = json_decode(file_get_contents($url), true);
$btcValue = $stats['market_price_usd'];
$usdCost = 200;

$convertedCost = $usdCost / $btcValue;

echo $convertedCost;
?&gt;     


http://phpfiddle.org/lite/code/cxsk-b62m     


  (If it helps, im a noob sorry)    


That's okay. If you're looking to learn more beginner related Bitcoin coding stuff check out this mega-thread at bitcointalk https://bitcointalk.org/index.php?topic=990348.0  It's mostly PHP and JavaScript examples.