This IS a bug: in Linux (and Unix like systems) beeing in a symlinked directory should behave exactly like beeing in a directory with the same content.

To use my example: If I use a shell and change to the folder /htdocs/test2 (which is a symlink to /test1), ls ../docs/docs.inc.php will show me the file "/htdocs/docs/docs.inc.php" and NOT "/docs/docs.inc.php".

I don't realy understand your problem ?!
[root@pix sdv]# ls -alR
.:
total 16
drwxr-xr-x  4 root     root    4096 Dec 17 17:44 .
drwxrwxrwx  3 via      ftponly 4096 Dec 17 17:44 ..
drwxr-xr-x  2 root     root    4096 Dec 17 17:45 docs
drwxr-xr-x  2 root     root    4096 Dec 17 17:46 test1
lrwxrwxrwx  1 root     root       5 Dec 17 17:44 test2 -> test1

./docs:
total 12
drwxr-xr-x  2 root root 4096 Dec 17 17:45 .
drwxr-xr-x  4 root root 4096 Dec 17 17:44 ..
-rw-r--r--  1 root root   24 Dec 17 17:45 docs.inc.php

./test1:
total 12
drwxr-xr-x  2 root root 4096 Dec 17 17:46 .
drwxr-xr-x  4 root root 4096 Dec 17 17:44 ..
-rw-r--r--  1 root root   50 Dec 17 17:46 index.php

[root@pix sdv]# cat test1/index.php 
<?php
include("../docs/docs.inc.php");
echo "ok\n";
?>
[root@pix sdv]# cat docs/docs.inc.php 
<?php
echo "docs\n";
?>

No problem when i try this with apache :
http://www.xxxx.com/sdv/test1/index.php
http://www.xxxx.com/sdv/test2/index.php
==> same output : docs ok 

if you try this in command line.
3 cases :
- pwd= test1 : php index.php => output docs ok
- pwd= test2 : php index.php => output docs ok
- pwd= anywhere else : php ./test1/index.php : include(): Unable to access ../docs/docs.inc.php which is quite normal
the include path is relative to the current directory where php is executed not relative to the php script which is executed ...

isn't it ?

No, you did not read my initial post correctly:
a file/directory starting with a / (like /test1) means it is in the root-directory. All example pathnames where absolute pathnames, not relative.

Here comes my listing for you:

/htdocs/:
total 12
drwxr-xr-x  3 root root 4096 Dec 18 22:23 ./
drwxr-xr-x 22 root root 4096 Dec 18 22:23 ../
drwxr-xr-x  2 root root 4096 Dec 18 22:23 docs/
lrwxrwxrwx  1 root root    6 Dec 18 22:23 test2 -> /test1/

/htdocs/docs/:
insgesamt 8
drwxr-xr-x 2 root root 4096 18. Dez 22:23 ./
drwxr-xr-x 3 root root 4096 18. Dez 22:23 ../
-rw-r--r-- 1 root root    0 18. Dez 22:23 docs.inc.php

/test1/:
total 8
drwxr-xr-x  2 root root 4096 18. Dez 22:24 ./
drwxr-xr-x 22 root root 4096 18. Dez 22:23 ../
-rw-r--r--  1 root root    0 18. Dez 22:24 index.php

Please note that test1 and test2 are not on the same directory level!

Thank you for this bug report. To properly diagnose the problem, we
need a short but complete example script to be able to reproduce
this bug ourselves. 

A proper reproducing script starts with <?php and ends with ?>,
is max. 10-20 lines long and does not require any external 
resources such as databases, etc. If the script requires a 
database to demonstrate the issue, please make sure it creates 
all necessary tables, stored procedures etc.

Please avoid embedding huge scripts into the report.

No feedback was provided for this bug for over a week, so it is
being suspended automatically. If you are able to provide the
information that was originally requested, please do so and change
the status of the bug back to "Open".

I confirm this bug. Here is the simplest reproduce code that I was able to make up:

~$ cd /tmp
/tmp$ mkdir a
/tmp$ mkdir b
/tmp$ echo '<?php echo getcwd(); require "../inc.php";?>' > a/test.php
/tmp$ echo '<?php echo "ok\n";?>' > b/inc.php
/tmp$ ln -s /tmp/a b/c
/tmp$ cd b/c/
/tmp/b/c$ php -f test.php
/tmp/a
Warning: require(../inc.php): failed to open stream: No such file or directory in /tmp/a/test.php on line 1

As one can see, the script's CWD is /tmp/a although the script was run from /tmp/b/c. As a result the following require() instruction fails because it can't find /inc.php (which should have been /tmp/b/inc.php instead).

The problem here is not the require(). It is that the CWD is converted to the real path when, instead, it should remain the symlinked path.