Change Random Number Every Hour ?

[dunlop03]

Im using this code to generate a random number:

<?php 
srand ((double) microtime( )*1000000); 
$random_number = rand(1000,5000); 
echo "$random_number"; 
?>

The problem is, I want this to generate a new random number every hour. So basically I want it to generate a number that lasts for 1 hour, then have it generate another number, automatically.

Is this possible, any ideas on how to do this?

Thank you in advance.