the & operator means "address of".
So if you have the variable $lang and its memory address is 0xAAAAAAAA
then using $tag_ = &$lang;
would actually create a reference to the memory location instead of actually creating a new variable and copying the contents.
when variables contain large amounts of data or big arrays or objects, using that is usually much faster that creating a new copy, however if you use the reference like that, and modify $tag, then the variable $lang will also change because it is being pointed to by $tag.
In a sense, you can think of $tag_ and $lang as being the same variable.
