On Nexen.net today, Damien Seguy presents some numbers on something that there's not a lot of research done on - the number of servers running multiple PHP versions.

This subject was hot at the birth of PHP 4 (running PHP 4 and PHP 3, or course), and it is coming back with a vengeance for PHP 5, backed by the immense user base of PHP 4. If PHP 5 is the default installation in 2008, what will happen to all those users? [...] Based on this observation and my earlier experience of cohabitation, I wondered if there was any indicator of multiple PHP installation.

He used the same tracker he implements for his normal monthly stats and ran a modified query to get the numbers of multiple-version servers that are out there. He's graphed some of the results. Based on his results, though, he notes a market for multiple PHP installations but "a pretty small one", noting that you're probably better off making the jump out to PHP5 anyway.