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<?php $array = []; $array["a"] =& $array["b"]; $array["b"] = 1; var_dump($array);
based on hCO9u
Output for 5.5.0 - 5.5.38, 5.6.0 - 5.6.40, 7.1.0 - 7.1.30, 7.2.0 - 7.2.33, 7.3.0 - 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 - 7.0.33
array(2) { ["a"]=> &int(1) ["b"]=> &int(1) }