Which PHP 5.X version should be considered as Required

"Possible old servers" where can be uploaded a new (2016 build) app with CI 4 sounds strange as a reason to keep down line of CI to is an old version as 5.4.

BTW such servers will be in a lot worse condition to match all issues with the PHP it self and other things as ssl and etc if they are not proper upgraded.

If I use Ubuntu LTS releases on my server, I get patches for security issues for 5 years, including security patches for most of the software. So, Ubuntu 12.04 LTS will still have PHP 5.3 when the OS reaches end of life in 2017. They may back-port security patches from newer versions of PHP to keep the server supported, but they won't back-port features or upgrade the version of PHP to 5.4+. Similarly, they'll release security patches for other software on the OS, like SSL.

Ubuntu 14.04 LTS (the latest LTS release) includes PHP 5.5, and will be in service until 2019. The next LTS release isn't expected until next year, so there are no Ubuntu LTS servers running a version of PHP newer than 5.5 unless they were upgraded through other channels. Ubuntu 10.04 LTS reached end of life fairly recently, but I'm not sure what version of PHP was included on that.

Other distributions may not follow the same 5-year life cycle, but most follow the same pattern of only updating the included software within a certain range of version numbers, so a given installation of the OS will never upgrade PHP 5.3 to 5.4 without upgrading the whole OS (though you can upgrade the software yourself, or add a 3rd party repository to your update software to do so, most server admins don't like to do that).

I have listed at the first post all difference in the version. PHP is growing in small steps if you don't want to jump often you have not to skip them as we have done that with CI 3.

Although CI 3 took too much time to be released else it was not going to be a big step next CI 4 version..
PHP 5.5 has a lot of features as generators, finally and etc..
PHP 5.6 has less new features but still there very nice new operator ... and it has a lot of improvements inside it and it should be focused as the BEST of PHP 5.

I'm aware of that, and even pretty familiar with the differences. When developing a framework, namespaces and closures have some pretty obvious benefits. The other features, to me, offer diminishing returns, especially since so many of them are adding more flexible syntax rather than adding completely new features.

The performance of a particular version of PHP should be irrelevant, as that benefit is available to anyone using that version, regardless of the minimum version supported by the framework.

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