I made mistake in the code - here is the corrected  
version:  
  
touch("foobar"); 
echo file_exists("foobar") . "\n"; 
 
mkdir("foodir"); 
chdir("foodir"); 
echo file_exists("foobar") . "\n";

Thank you for taking the time to write to us, but this is not
a bug. Please double-check the documentation available at
http://www.php.net/manual/ and the instructions on how to report
a bug at http://bugs.php.net/how-to-report.php

Use clearstatcache() to clear the cache.

Don't you think that the key that is used to access the 
cache should be the _full_, _absolute_ path instead of 
just the string passed to file_exists()? 
 
Current solution doesn't guarantee proper results - how do 
you know whether the code you call doesn't change into 
some other directory and stats() eg. "foo" won't interfere 
with the other piece code checking for "foo" existence in 
other directory? The only way to do such checks reliably 
is to call clearstatcache() _each_ _time_ what makes the 
cache useless. At least for me. I think it's a design 
error.

No, I don't think so.
If you're trying to outsmart the cache jumping from/into different directories and doing the stat() using *relative* path - this is not PHP problem (and it can be easily solved with clearstatcache(), but I've already told you that).

To "outsmart the cache"... Rest in piece, Unix principles. 
 
Hint for those who encounter the same problem as mine: use 
realpath(). A hack, but works as file_exists() should.