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run code in 200+ php & hhvm versions
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<?php $first = array( 'abc' => array() ); $alias = '30000'; var_dump(array_merge($first, array($alias => array())));
based on n68i8
Output for 4.3.0 - 7.2.0
array(2) { ["abc"]=> array(0) { } [0]=> array(0) { } }