This tag is used for questions that involve PHP code in any way. So as soon there is a PHP Exception returned or error report generated, this question qualifies to be tagged with PHP.
PHP is the Programming Language Magento is written in, at least the part of Magento which runs directly on the Server.
Most of the users of the Magento Stack Exchange are developers who have some or good knowledge of PHP and may expect the same from you. So try to always get all the needed information if you have a PHP related problem. Try to get error messages as detailed as possible, the stack trace is also often helpful (the many lines of file paths usually stored in the error report or the exception.log) or explicit code examples describing what you are working on when you are a programmer yourself.