The upgrade to PCRE2 should mostly be transparent to existing code, but
apparently there are some minor differences, which warrant a note in
UPGRADING (and consequently, in the migration guide).
. Support for ODBCRouter has been removed.
. Support for Birdstep has been removed.
+PCRE:
+ . The PCRE extension has been upgraded to PCRE2, which may cause minor
+ behavioral changes, and augments the existing regular expression syntax.
+ See <https://wiki.php.net/rfc/pcre2-migration> for details.
+
ZIP:
. Bundled libzip has been dropped,
system library is now required.