<?php
$arr = array('one' => ['four' => 4], 'two' => '2', 'three' => '3');
var_dump($arr);
$arr2 = array_flip($arr);
var_dump($arr2);
?>

The above example will output:
array(3) {
 ["one"]=>
 array(1) {
 ["four"]=>
 int(4)
 }
 ["two"]=>
 string(1) "2"
 ["three"]=>
 string(1) "3"
}
Warning: array_flip(): Can only flip STRING and INTEGER values! in /root/test.php on line 4
array(2) {
 [2]=>
 string(3) "two"
 [3]=>
 string(5) "three"
}