simple if then statement

I have very little knowledge of php. I am trying to put together a very simple script everything is working it is very short and simple however. I am utilizing a get function to pull images from a website. I would like to only get the image/images if it is older then 15 minutes. My current code is

<?php 
$weatherradar = file_get_contents('http://radar.weather.gov/Conus/Loop/northeast_loop.gif');

$fp = fopen('image.gif', 'w');
fwrite($fp, $weatherradar);
fclose($fp); 
 ?>

PHP:

Now It should be a simple bit of code to use but I have know idea of what the code would be, any help would be greatly appreciated. Basically I just want to only get and replace the image if it is older then 15 minutes.

Thanks in advance for any help.

 

<?

$fn = 'image.gif';
if (!file_exists($fn) || filemtime($fn) + 15*60 < time())
{
 //insert get file code here
}

?>

PHP:

 

thanks a lot, you saved me a great deal of time believe it or not. It is not that I am too lazy to look it up it just takes forever to find what you are actually looking for.
Thanks again