- var_dump: documentation ( source)
- array_flip: documentation ( source)
<?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));