Description:
------------
Strings can be incremented by the ++ operator but they don't get decremented by the -- operator. This is seems inconsistent and it is also not documented in the manual.  

Test script:
---------------
<?php
$test = 'abc12';

// increment works as expected
echo(++$test . "\n");
echo(++$test . "\n");
echo(++$test . "\n");

// decrement fails
echo(--$test . "\n");
echo(--$test . "\n");
echo(--$test . "\n");

Expected result:
----------------
abc13
abc14
abc15
abc14
abc13
abc12


Actual result:
--------------
abc13
abc14
abc15
abc15
abc15
abc15