3v4l.org

run code in 200+ php & hhvm versions
<?php $array = []; $array['a'] =& $array['b']; $array['b'] = 1; var_dump($array);
based on ZiJfv
Output for 5.6.0 - 5.6.26, 7.1.0 - 7.2.6
array(2) { ["b"]=> &int(1) ["a"]=> &int(1) }
Output for 7.0.0 - 7.0.20
array(2) { ["a"]=> &int(1) ["b"]=> &int(1) }