Migrating PECL extension : Advice needed

Hi everybody, I'm currently adapting the 'PHK' PECL extension for PHP 7. I initially wanted to keep a single branch for PHP 5 and PHP 7, and enclose differences in macros and #ifdef blocks, starting with a backport of zend_string. Unfortunately, PHK interacts a lot with the PHP core and, as changes accumulate, I see my code become less and less readable. So, I now think I'll have to create a separate branch. As many of you probably had to make the same decision, can you tell me how you numbered your versions for the 2 parallel branches ? Use a different major number ? minor number ? same version with different suffixes ? Thanks and regards François