Description:
------------
This is not a bug, but I propose a change to how array_flip operates with 
boolean values, in order to improve common use cases, because:

1) The current behavior (warning and completely empty array) renders arrays with 
one or more boolean values invalid for flipping so they have to be manually 
filtered in advance.

2) When true/false are cast to 1/0, all loose type checks against the keys will 
match (i.e. true == 1, false == 0) which makes the cast also a practical one for 
real utility.

3) The change will make the casting behavior of array_flip consistent with the 
*already* existing behavior in the following example:

// casts booleans to int, and produces: array(1 => 123, 0 => 321);
$a = array(); $a[true] = 123; $a[false] = 321;




Test script:
---------------
var_dump(
    array_flip(
        array('foo' => true, 'bar' => false)
    )
);

Expected result:
----------------
array(2) { [1]=> string(3) "foo" [0]=> string(3) "bar" }

Actual result:
--------------
Warning: array_flip() [function.array-flip]: Can only flip STRING and INTEGER 
values! in ... on line 2

array(0) { }