Gary over at thephpgrind.net posted about slow PHP5 adoption. I posted a couple thoughts on why PHP5 hasn’t been adopted as quickly as people would like, but forgot another reason: concurrency.
When going from PHP3->PHP4, it was possible to run both mod_php3 and mod_php4 in Apache at the same time – .php3 files would be run under PHP3, and .php files could run under PHP4. You can not do this with PHP5, which I think has been another reason why people have been slow to adopt it. You can’t upgrade at your own pace – your entire app has to be upgraded at one time. Control panel providers like Plesk and Ensim would quickly provide PHP4 and 5 simultaneous running, which would allow millions more to upgrade and plan a migration, but it’s simply technically not possible. This is a huge hindrance, and will continue to be until someone comes up with a way to run both at the same time (both as modules, not a modphp/cgi mix).
