That's 63.5% of all PHP5 questions
So I suggest merging those 133 questions into PHP and deleting the PHP5 tag. There is already a PHP4 tag (albeit a small one) but that can be used to distinguish between legacy code and current code.
There are questions though that fall in the category of comparing versions of PHP 4 and 5 and upgrade questions from 4 to 5. But most of the questions I see are questions that are tagged PHP PHP5.
Same way that there is no python2 tag but there is python3.0 tag and a python tag. It's inconsistent.
Another example. There are 14 questions that are tagged PHP5 and PHP4 and they are all but 1 or 2 that are about compatibility issues / feature that was in the old version or migration from 4 to 5.
There are 7 questions that could pose a problem. Since they are tagged PHP4 PHP5 but not PHP