Description:
------------
it is possible to overwrite defined variables

Reproduce code:
---------------
define('FOO','bar',true);
echo FOO; // will print "bar"

define('FOO','bar',true);
define('FOO','stuff',true);
echo FOO; // will print "bar"

define('FOO','bar',true);
define('FOO','stuff'); // <<< here is the bug
echo FOO; // will print "stuff"

Expected result:
----------------
FOO won't be overwritten

Actual result:
--------------
it will printed out "stuff"