3v4l.org

run code in 500+ PHP versions simultaneously
<?php $a = new stdClass; $a->{'abc@gmail.com'} = []; $b = new stdClass; $b->{'xyz@gmail.com'} = []; $c = [$a, $b]; $arr = array_reduce($c, function ($arr, $obj) { return array_merge($arr, array_keys(get_object_vars($obj))); }, []); var_dump($arr);
Output for 7.2.0 - 7.2.34, 7.3.0 - 7.3.33, 7.4.0 - 7.4.33, 8.0.0 - 8.0.30, 8.1.0 - 8.1.34, 8.2.0 - 8.2.35, 8.3.0 - 8.3.36, 8.4.1 - 8.4.27, 8.5.0 - 8.5.11
array(2) { [0]=> string(13) "abc@gmail.com" [1]=> string(13) "xyz@gmail.com" }

preferences:
38.74 ms | 1506 KiB | 7 Q