Johannes Schlüter has a post to his site that talks about references in PHP 7 and how they're "still bad" based on some of his previous findings.

I'm known for telling "Don't use references" (also as video) as those cause different problems (i.e. with foreach) and hurt performance. The reason for the performance loss is that references disable copy-on-write while most places in PHP assume copy-on-write. Meanwhile we have PHP 7. In PHP 7 the internal variable handling changed a lot among other things the reference counting moved from the zval, the container representing a variable, to the actual element. So I decided to run a little test to verify my performance assumption was still valid.

He includes his testing code that calls a function (strlen) in a loop and compares the handling against two methods, one passing by reference the other not. The results are shown in time taken to execute. He compares the results for PHP 5 and PHP 7, noting that PHP 7 is marginally better when passed by value, by-reference is still about the same.