PHP developers cannot leverage PHP 5's full potential without dropping support for PHP 4, but PHP 4 is still installed on a majority of shared web hosts and users would then be forced to switch to a different application. Web hosts cannot upgrade their servers to PHP 5 without making it impossible for their users to run PHP 4-targeted web apps, and have no incentive to go to the effort of testing and deploying PHP 5 while most web apps are still compatible with PHP 4 and the PHP development team still provides maintenance support for PHP 4. The PHP development team, of course, can't drop maintenance support for PHP 4 while most web hosts still run PHP 4. [tags] web apps, php developers, web hosts, maintenance support, php development, application web, php 4, leverage, servers [/tags]