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Semaphore
PHP Manual

# Predefined Constants

The constants below are defined by this extension, and will only be available when the extension has either been compiled into PHP or dynamically loaded at runtime.

System V message constants
Constant | Type | Changelog
--- | --- | ---
MSG_IPC_NOWAIT | integer |
MSG_EAGAIN | integer | As of 5.2.0
MSG_ENOMSG | integer | As of 5.2.0
MSG_NOERROR | integer |
MSG_EXCEPT | integer |