Race condition in the symlink function in PHP 5.1.6 and earlier allows local users to bypass the open_basedir restriction by using a combination of symlink mkdir and unlink functions to change the file path after the open_basedir check and before the file is opened by the underlying system as demonstrated by symlinking a symlink into a subdirectory to point to a parent directory via .. (dot dot) sequences and then unlinking the resulting symlink.