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run code in 200+ php & hhvm versions
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<?php $a = ['properties' => ['test']]; $b = ['activities' => [1]]; array_push($a['properties'], $b); var_dump($a);
based on cl62Q
Output for 5.6.0 - 7.3.0alpha1
array(1) { ["properties"]=> array(2) { [0]=> string(4) "test" [1]=> array(1) { ["activities"]=> array(1) { [0]=> int(1) } } } }