PHP curly braces in array notation

I'd just come across a very weird bit of php code:

$oink{'pig'} = 1;
var_dump($oink);

$oink{'pig'} = '123123';
echo $oink{'pig'}; /* => 123123 */
echo $oink['pig']; /* => 123123 */

It works like an array, but nowhere mentioned in the manual. What is this?