See the revision history on this question, where the beginner tag was edited in (and subsequently edited out after I started this discussion). I think, by adding this tag, you're making assumptions about the OP which they could find a little bit offensive. Particularly with this question, there's no mention of the level of PHP knowledge — it's more a math question than it is a programming question.
Someone could be excellent at a particular language whilst not being particularly great at math, so isn't it a bit insulting to make the assumption they're a beginner?
It's also interesting to note the question count for the tag tag, which currently stands at 7,784 on SO. Compare this to expert, which only has 18 questions, most of them being "discussiony." Why do users retag questions as "beginner" but not "expert"?