How could a "tag" appear here?

Question

PHP

Hello. I am making a ChekURL to accept a response from Yandex Checkout. And the return response from my side in the request contains something like a head tag.
Here's what I get: And here is the code itself: Why is this even possible?
<?xml version="1.0" encoding="UTF-8"?><head/>
print '<?xml version="1.0" encoding="UTF-8"?>';