These were desynonymized due to this question:
Clearly there isn't support for undoing that for policy reasons, so I'll try a different argument:
PHP 5 is the current version of PHP, and has been for the past 7 years. In the abstract, policy-level world, maybe
php aren't synonyms. In actually curating SO content, they are.
If anyone were to go through the ~4000 posts tagged
php5 and retag to
php those questions which are not about anything version specific, they'd be retagging 99% of them. In the other 1%, the
php5 tag doesn't add anything anyway, since PHP5 is the default PHP askers/answerers are talking about.
It's not like other languages where there is any significant number of people using a previous version and tagging with version numbers actually clarifies what you can use in your answer. Not tagging these questions as
php is just throwing away value. It makes them less visible to SO users, and less likely to match Google/Bing search queries.
Optimally someone would come along and retag all those questions, but PHP users are so... special... that there are too many mistagged to clean up.