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<?php $a=array("class" => "bal"); $a += array( "c" => "d" ); $a += array( "e" => "f" ); print_r($a); ?>
Output for 5.6.0 - 5.6.30, hhvm-3.15.4, 7.0.0 - 7.3.0beta1
Array ( [class] => bal [c] => d [e] => f )