Philadelphia has a couple of fares to Europe under $600 today with Dublin $528 round trip nonstop and Paris $593 round trip nonstop flights. Low fare to Dublin is available from Sep 11 all the way into May 2017. Paris CDG nonstop flights for $593 round trip are available every date Sep 16 to Oct 29, then rise to upper $700s, until March 16 when they once again drop to $593 round trip most dates into May 2017.
Portugal and Spain are on sale from late August through October. Lisbon ($610-$629 nonstop) and Porto ($627-$643 via Madrid) low fares may be motivated by TAP Portugal low fares across Spain from Miami. AA has lowered fares to Barcelona ($685) and Madrid ($690) too.
There are a few other Eastern European cities on sale with Krakow, Poland $628 to $657 late August to early Dec. Budapest, Hungary $666 – $688 late August to early Dec and Sofia, Bulgaria $676 – $680 late August to early Dec.
American Airlines Philadelphia PHL – Dublin, Ireland DUB
$527.46 round trip Tue Sep 20 – Tue Sep 27
American Airlines Philadelphia PHL – Paris CDG
$592.86 round trip Fri Sep 16 – Sun Sep 25
American Airlines Philadelphia PHL – Lisbon LIS
$609.76 round trip Tue Aug 23 – Wed Aug 31