Установка

Support for this functions are not enabled by default. To enable System V semaphore support compile PHP with the option --enable-sysvsem . To enable the System V shared memory support compile PHP with the option --enable-sysvshm . To enable the System V messages support compile PHP with the option --enable-sysvmsg .

Коментарии

It should be mentioned that depending on the system, an module can be available.

On CentOS I just ran:
sudo yum install php73-php-sysvsem

then reload:
sudo service php-fpm reload (or apache, depending on your case)

and semaphore functions are now available.
2020-04-05 16:29:06
http://php5.kiev.ua/manual/ru/sem.installation.html

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