More precisely it needs >=dev-lang/php-5.3.3, but for now it nailed up to php:5.3. Quote from in-tree ebuild: "=dev-lang/php-5.3*[${DB_FLAGS},${AUTHENTICATION_FLAGS},${PHP_FLAGS}]" I personally use it with php-5.5 and it works fine.
Fixed in the tree. Please reopen if this is still an issue: @@ -39,7 +39,7 @@ # which may live on another server DEPEND="" RDEPEND=" - =dev-lang/php-5.3*[${DB_FLAGS},${AUTHENTICATION_FLAGS},${PHP_FLAGS}] + >=dev-lang/php-5.3.3[${DB_FLAGS},${AUTHENTICATION_FLAGS},${PHP_FLAGS}] virtual/httpd-php virtual/cron"