Here is how I did it on my old mining pool website with PHP and mySQL from TradeHill, which is *very* similar to how you would do it with MtGox, unless you want to use websocket/sockets.io for live AJAX stuff.
$file = fopen("https://api.tradehill.com/APIv1/USD/Ticker", "rb");
$tradedata = fread($file, 8192);
$jsonTradedata = json_decode($tradedata, true);
mysql_query("UPDATE `websiteSettings` SET `tradeHillWorth` = '".$jsonTradedata['ticker']['last']."'");
This accessed a table in MySQL called websiteSettings in my main DB. You don't need a database, but it makes it easier to reduce load on your website because you can pool every N times, instead of every time the page loads.
Then you run this somewhere in your PHP code before you want to call the variable:
$tradeHillQ = mysql_query("SELECT `tradeHillWorth` FROM `websiteSettings`");
$tradeHillWorth = mysql_fetch_object($tradeHillQ);
Then you run this when you want to insert the price in the output of the HTML page:
<?php echo round($tradeHillWorth->tradeHillWorth, 2);?>
The code above will round it to two decimal places. Again, this is very, very similar to how you would do it for any exchange. Just read their API documentation.