`phpversion()` return value and `PHP_VERSION` constant value are identical, but the latter is several times faster because it is a direct constant value lookup compared to a function call.
Props ayeshrajans, jrf, mukesh27, costdev, hellofromTonya, SergeyBiryukov.
Fixes#55680.
git-svn-id: https://develop.svn.wordpress.org/trunk@53426 602fd350-edb4-49c9-b593-d223f7449a82