dev-lang/php produces an ewarn level message stating "Enabling ZTS for Apache2 MPM". I'm sure this warning is there for good reason, but the message should contain some detail, even if just a link, with more information on why this is a warning (ie. why is it important) and what, if anything, users should do about it.
Sorry for getting to this so late: the ewarn is gone in more modern versions (>5.2.10) and probably an einfo message would have served the same purpose: to tell you, that you're compiling the threaded version of php.