How do I store random number

My PHP-knowledge is limited, so maybe this is a very simple question.
Is there a way to store a random number on the server for a limited time.
The scenario is this.
I have a flashmovie, that is communicating with various php-scripts.
As a start the flashmovie validates username and password through checklogin.php.
As part of the validation-proces I want to send a random number or security-word back to the flash-movie.
When the flash-movie then communicates with another php-scripts - like createxml.php, I want it to send the random-number as a securitymeasure before the actual php-script executes.
if ($flash-number==$server-number)
But that takes that the random number is stored either in an included file or in some other manner.