Luckily, the PHP documentation team provides you with a set of migration guides. Going through these guides can take some time, but it enables you to upgrade your code easily to be PHP5 compatible.
To make your life even easier, the PHP-tools project is extended with the tool test-migration. This tool performs some checks that are described by the migration guide to detect whether the code can be run under version 5 of PHP. These checks include:
- Is there a function definition with the same name as a function newly defined in PHP5?
- Is an object created without the class being defined first?
- Where are the functions strrpos, strripos and ip2long used?
- Is there any place in which there is reflection within PHP that uses changed behavior?
On the other hand, the last two checks do not find constructions that can halt execution. They detect places in which certain constructions are used. These constructions where already available within PHP version 4, but their behavior changed in version 5. To easily find these constructions they are flagged by a 'minor'-warning. More details about the changed behavior can be found here.
The first version of this tool is quit basic and performs only a few checks. If you would like to see more check included, don't hesitate to drop me an email or put them in the comments.