Description:
------------
file_exists() etc. does not allow checking the existence of files outside the directories specified in open_basedir.

Appearently readfile() does *not* have this restriction and thus allows checking the existence of files anywhere in the filesystem. If realpath($filename) returns a string (i.e. not false), the file exists. This is a circumvention of the open_basedir restriction in file_exists() etc.


Reproduce code:
---------------
The following should always be true, but it isn't when $dir is outside the directories specified in open_basedir:

var_dump(file_exists($dir) === (bool) realpath($dir));