Implementation of the finished product is often a laborious task, however PHP's server-side nature and centralised php.ini configuration mean that most applications you write can be deployed with relative ease. There are still things to consider, however: how do users create the initial database and set up passwords? What changes need to be made to their php.ini file, if any? Should you check for a particular version of PHP? Does your application only work on one platform? Does it specifically require a certain module not enabled by default?
While a readme file is often good enough, you will probably find the best bet is to have a configuration script that can be run by the end user, which performs configuration for them. Some products on Unix, for example, have initialisation scripts that, by default, terminate the application with the message "You have not edited your configuration" - users need to edit their config file, at the very least, to remove the automatic exit command, but that requirement also gives them their best opportunity to make other required changes while there.
Although PHP is usually shipped with two example php.ini files, a standard one and a recommended one, you should generally aim for compatibility with only the recommended file, as it is stricter than the standard configuration. You are almost certainly going to cause problems if you insist on using short tags for example, so it is in your own interest to stick to the standard configuration.