Document best practices for updating to new PHP versions

[dktapps]

There's a few tricks I picked up over the years to figure out what needed to be changed in new versions:

• git diff PHP-8.2..PHP-8.3 -- php-src/ext/opcache/zend_persist.c (replace 8.2 and 8.3 with the old & new version respectively) is a great source of info for what needs fixing for class & function copying
• Test ALL new language features. Pretty much every time a new feature gets added, it breaks in threads because there was some new data associated with it.
• Build & run tests locally with AddressSanitizer. This often catches errors that Valgrind doesn't see.

to be continued...