3v4l.org

run code in 300+ PHP versions simultaneously
<?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 - 7.1.20, 7.2.0 - 7.2.33, 7.3.16 - 7.3.27, 7.4.0 - 7.4.15, 8.0.0 - 8.0.2
array(2) { ["b"]=> &int(1) ["a"]=> &int(1) }
Output for 7.0.0 - 7.0.20
array(2) { ["a"]=> &int(1) ["b"]=> &int(1) }