the problem with 90 degrees in Philly is that it's routinely accompanied by very high humidity as well. This is a dangerous combination and is definitely a different type of hot than is encountered in AZ, for example. I do not live in Philly but farther north and we don't get the super hot days as much.
Of course, I try to ride or exercise when it is hot, although I think that fluid intake should be very important and monitored closely.