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<?php $a = array('a' => "test", 'b' => 'aa'); $a['a'] = array(); $a['a'] += array('bb' => "gdaf"); var_dump($a); ?>
based on m7GvW
Output for 4.3.0 - 7.2.0
array(2) { ["a"]=> array(1) { ["bb"]=> string(4) "gdaf" } ["b"]=> string(2) "aa" }