(PHP 4, PHP 5)
sem_acquire — Acquire a semaphore
sem_acquire() blocks (if necessary) until the semaphore can be acquired. A process attempting to acquire a semaphore which it has already acquired will block forever if acquiring the semaphore would cause its maximum number of semaphore to be exceeded.
After processing a request, any semaphores acquired by the process but not explicitly released will be released automatically and a warning will be generated.
sem_identifier is a semaphore resource, obtained from sem_get().
成功时返回 TRUE， 或者在失败时返回 FALSE.