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run code in 300+ PHP versions simultaneously
<?php $a = [ 'first' => 4, 'second' => 5, 'third' => 6 ]; $b = [ 'first' => 6, 'second' => 7, 'forth' ]; var_dump($a + $b);
Output for 7.1.0 - 7.1.20, 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(4) { ["first"]=> int(4) ["second"]=> int(5) ["third"]=> int(6) [0]=> string(5) "forth" }

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