Description:
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When pcntl_signal is passed a null variable instead of a callable, the error message should make it clear that the variable is not allowed to be null. Currently the error message given is:

> 'pcntl_signal(): is not a callable function name error'

Presumably because it is converting the variable null to be a string of ''. 

Which means the developer needs to work backwards from a zero length string being present in the error message, to the fact that they passed in null.

Test script:
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static $fnSignalHandler = null;

// $fnSignalHandler is meant to be initialised here but isn't

pcntl_signal(SIGINT, $fnSignalHandler, false);

Expected result:
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A clear error message that says I've passed null.

Actual result:
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Me spending 15 minutes trying to figure out why pcntl_signal is not callable.