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Core PHP Programming: Using PHP to Build Dynamic Web Sites by Leon Atkinson

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Semaphores

PHP offers an extension for using System V semaphores. If your operating system supports this feature, you may add this extension to your installation of PHP. At the time of this writing, only the Solaris, Linux, and AIX operating systems were known to support semaphores.

Semaphores are a way to control a resource so that it is used by a single entity at once, and they were inspired by the flags used to communicate between ships. The idea to use an integer counter to ensure single control of a resource was described first by Edsger Dijkstra in the early 1960s for use in operating systems.

A complete tutorial on semaphores is beyond the scope of this text. Semaphores are a standard topic for college computer science courses, and you ...

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