Can't require 'config.php'? -

is config.php reserved name or why following not working me?

i have following files:

/test.php:

echo "test<br/>"; require("php/session.php"); 

/php/session.php:

echo "session<br/>"; require("connect.php"); 

/php/connect.php:

echo "connect<br/>"; require("config.php"); 

/php/config.php:

echo "config<br/>"; 

if open /test.php, prints test, session , connect. if rename config.php else , change require in connect.php, works.