The Hardened-PHP Project has released another vulnerability today, this time it's an issue with one of PHP's own internal functions - open_basedir.
The design of the open_basedir feature of PHP that is meant to disallow access to files outside a set of configured directories is vulnerable to race conditions.
It was discovered that this design flaw can be exploited with the usage of PHP's symlink() function in a very easy way. We believe that the only solution to this problem is disabling the function symlink() while open_basedir is used (this feature was therefore added to our Suhosin PHP Security Extension).
They also note, unfortunately, that the problem may not be fixable due to how it can be implemented. They provide a more detailed explaination and some PHP psuedo-code to help illustrate the point.