From #1124 (comment):

It will cause following problems when building php-5.6.40. If I change the statement to $build->compareVersion('7.4') fixed the problem.

make '-j8' >> '/Users/mark/.phpbrew/build/php-5.6.40/build.log' 2>&1
Error: Make failed:
The last 5 lines in the log file:
3 errors generated.

make: *** [ext/mbstring/php_mbregex.lo] Error 1

make: *** Waiting for unfinished jobs....

2 warnings generated.

1 warning generated.

Please checkout the build log file for more details:
	 tail /Users/mark/.phpbrew/build/php-5.6.40/build.log