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<?php $a[] = 'd'; $a['d'][] = 4; $a['d'][] = 6; var_dump($a);
Output for 5.5.24 - 5.5.35, 5.6.8 - 5.6.28, 7.0.0 - 7.0.20, 7.1.0 - 7.1.10, 7.2.0 - 7.2.33, 7.3.16 - 7.3.33, 7.4.0 - 7.4.33, 8.0.0 - 8.0.30, 8.1.0 - 8.1.27, 8.2.0 - 8.2.17, 8.3.0 - 8.3.4
array(2) { [0]=> string(1) "d" ["d"]=> array(2) { [0]=> int(4) [1]=> int(6) } }

preferences:
158.45 ms | 404 KiB | 177 Q