php process fifo queue

The task is to find a way that enables php calls to an external application to be processed one at a time in the order they were entered. The context here is that I have, on a web server, an application that is being called by a PHP script. The execution of the application is quite costly in terms of CPU performance. The whole process is initiated by users (visitors to the homepage) so there may be several simultaneous calls. Unchecked execution of these calls may result in undesirable behaviour. Therefore I need a fifo queue for <em>every</em> PHP process on the system that wants to call this external application.