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<?php $a =5; $a = array(&$a); var_dump($a);
Output for 8.0.0 - 8.0.30, 8.1.0 - 8.1.27, 8.2.0 - 8.2.18, 8.3.0 - 8.3.6
array(1) { [0]=> *RECURSION* }
Output for 5.2.14 - 5.2.17, 5.3.3 - 5.3.29, 5.4.0 - 5.4.45, 5.5.0 - 5.5.38, 5.6.0 - 5.6.40, 7.0.0 - 7.0.33, 7.1.0 - 7.1.33, 7.2.0 - 7.2.33, 7.3.0 - 7.3.33, 7.4.0 - 7.4.33
array(1) { [0]=> &array(1) { [0]=> *RECURSION* } }
Output for 4.3.0 - 4.3.11, 4.4.0 - 4.4.9, 5.0.0 - 5.0.5, 5.1.0 - 5.1.6, 5.2.0 - 5.2.13, 5.3.0 - 5.3.2
array(1) { [0]=> &array(1) { [0]=> &array(1) { [0]=> *RECURSION* } } }

preferences:
261.54 ms | 402 KiB | 459 Q