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run code in 200+ php & hhvm versions
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<?php $array = array( 0 => array('user_id' => '1', 'email'=>'email1'), 1 => array('user_id' => '2', 'email'=>'email2'), 2 => array('user_id' => '3', 'email'=>'email3') ); //var_dump(array_flip($array));
Output for 5.3.0 - 7.2.0