Outdated source code
We just finished upgrading PHP for a client from version 5.3 to 5.6. Even they had sent out notifications to their customers in advanced, multiple websites were not working after upgrading. The reason is that of outdated source code, which is incompatible with PHP version 5.6.
There are reasons for PHP to change their core functions: performances, securities or even just because the database developers change their structure. After each major release, some functions are deprecated and replaced by the new ones.
A typical user, or entrepreneur, might not know about those changes or they do not know how to upgrade their code. Those users will need to have good developers who pro-actively perform a full check and replace deprecated functions, to make sure the source code is working well with the new platform, or even running better.
Now, you might know that PHP has just released version 7.0 with big changes in performance. You want to have that, but is your website code ready yet?