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<?php $array = array( 0 => array('user_id' => '1', 'email'=>'email1'), 1 => array('user_id' => '2', 'email'=>'email2'), 2 => array('user_id' => '3', 'email'=>'email3') ); var_dump(array_keys($array));
Output for 4.3.0 - 7.1.0
array(3) { [0]=> int(0) [1]=> int(1) [2]=> int(2) }