When purchasing an online ticket, how do you indicate that you want to stopover in a particular city?
For example, outward bound: Philadelphia to London to Prague. On the homeward flight, after leaving Prague, I want to stay several days in London before returning to Philadelphia.
Jim - Check multi-city options on whatever airline or booking engine you are using. You can specific cities and dates for each flight. We have found when flying through London, it's often cheaper to book open jaw - into Prague and out of London, purchasing your Prague to London flight separately. Sometimes just a round trip to London with separate tickets to and from Prague is a cheaper option.
Don't know the exact rule but if you break a flight more than about 24 hours it becomes a separate flight. Some airlines - Icelandic Air for example - will offer extended layovers as a promotional package but that is infrequent. As suggested above, use the multi city option and book a three leg flight. The additional cost is generally small. We just booked a five leg flight over a three week period which was only slightly more than an original round trip that we looked at ,
Fares also vary quite a bit depending on which city you're flying to/from. I just priced tickets that were noticeably cheaper flying home from Budapest rather than Vienna, even when I had to change planes in Vienna... Taxes make a difference too, Heathrow has notoriously high taxes - but is still cheaper than Paris for the trip I'm pricing.