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<?php $a = array('a' => "test", 'b' => 'aa'); $a = array_merge($a, array('aa' => "dfd")); var_dump($a); ?>
based on mKLlu
Output for 4.3.0 - 7.2.0
array(3) { ["a"]=> string(4) "test" ["b"]=> string(2) "aa" ["aa"]=> string(3) "dfd" }