When I am developing programs that might be utilized on client web serves of various designs, frequently with hosting that is shared, which PHP 5 version must i assume most web servers have, e.g 5.2.x, 5.3.x, etc?

Version 5.2 can be used by 84.9% of all of the websites who use PHP version 5.

Version 5.3.3 can be used by 39.7% of all of the websites who use PHP version 5.3

You can observe more record data in the link that follows:

http://w3techs.com/technologies/particulars/pl-php/5.3/all