php - How exactly is this function a call-by-reference function?

I get this warning message:

PHP Notice:  Only variables should be assigned by reference in /home/sophia/gitter/project/rational-office-2019/tmp/zog.php on line 16
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What is in line this 'zog.php' file?

$the_mainrdn = &obtaino($zogcn,'mainrdn');
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So, you would think from this warning that the obtaino function would have a second argument that is supposed to be called by reference - but here is the start and end of the function in question:

function obtaino ( $the_xcont, $the_clnom )
{
  $awesome_dom = new DOMDocument();
  $awesome_dom->loadHTML($the_xcont);
  -- <some more stuff here> --
  return $awesome_bdv;
}
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as you can see, there is no & symbol before the $the_clnom at the opening of the function - so I do not see how this is call by reference.

So what am I missing?