=& in footnotes

[tacman]

I've been running phpstan level 6, and see this code

		if (isset($this->footnotes[$node_id])) {
			$num =& $this->footnotes_numbers[$node_id];
			if (!isset($num)) {

I'm wondering what the purpose of =& is. Also, I think we can simply say

if (empty($num))

since $num is always set and it can't be null. But I'm not sure.

[michelf]

It's supposed to create a reference to $this->footnotes_number[$note_id] so that if you write to $num it'd also change the other place. So this other line is actually assigning to $this->footnote_number[$note_id], and the isset($num) is checking for the existence of $this->footnote_number[$note_id].

Note: In many places in the code I'm using =& simply as a way to avoid warnings about unset variables, but this is not one of those cases.