Do not accept input from end users which will be directly interpreted as source code. If this is a business requirement, validate all input to the application by removing any data that could be directly interpreted as PHP source code.
This vulnerability is not difficult to leverage. PHP is a high level language for which there are vast resources available. Successful exploitation requires knowledge of the programming language, access to the source code or the ability to produce source code for use in such attacks, and minimal attack skills.