<explain>
This example installs an alarm signal, but the callback function does not exist,This will set the last error. Then we will print out the last error.
<?php
set_error_handler(function(){});
pcntl_signal(SIGALRM,'sig_handler',false);
print 'Your error returned was (Code):'.pcntl_get_last_error().PHP_EOL;
print 'Your error info:'.pcntl_strerror(pcntl_get_last_error()).PHP_EOL;