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POSIX defines so called real-time signals which are essentially signals without a specific meaning that are free to be used for any purpose.
These signals have numbers in the range from SIGRTMIN to SIGRTMAX and there are usually ~30 of them available (see http://man7.org/linux/man-pages/man7/signal.7.html for more information on real-time signals).
The required constants SIGRTMIN and SIGRTMAX are currently not available in PHP. In addition, pcntl_signal() has a hard coded upper limit of 32 for signal values so no signal handlers for these real-time signals can be registered even if they are passed in as numeric values (which is dangerous as some of the signals are used by the glibc POSIX threads implementation and the SIGRTMIN constant is adjusted dynamically to reflect that).
I have a pull request open at https://github.com/php/php-src/pull/1457 to implement that missing behaviour.
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There is a pull request to fix this issue which has been ignored.
This was implemented in 7.2 I believe.