Many older sites written in PHP as well as many newer sites that evolved from older legacy code will find their sites broken when their servers are upgraded to PHP 5.3. That’s because many older PHP functions and INI directives are now formally deprecated (made obsolete) in version 5.3.
This will not just affect ancient web sites written with very old versions of PHP. Many fairly current PHP sites, including several popular shopping carts, will crash when their hosting company upgrades the server to PHP 5.3. As a code system evolves, it is not unusual for some older code to remain.
We learned about the PHP 5.3 through the Zen Cart forum. The Zen Cart team has already produced a patch consisting of several scripts that need to be replaced to upgrade the code. Kudos to these guys and gals, because they are on the ball, which will be greatly appreciated by many loyal Zen Cart users.
Most of the time deprecated functions continue to work when a PHP version is upgraded, because older functions are typicaly grandfathered in when a new version is released. Apparently this is not the case with with PHP 5.3 because a Deprecated Error (not a warning) is generated. This can cause a site to fail.
The PHP site does display a list of depreciated functions along with the newer equivalent replacements. If you have sites written in PHP, it may be wise to scan through this list as well as your PHP code.