PHP not parsed if sitting outside the doc root

Dear all,

I have a library of php files that I would like to use in several web sites.

I thus have several directories, one for each web site, containing the different web sites files.

One web site is at:
/home/httpd/vhosts/thalasoft.net/httpdocs/

Another one is at:
/home/httpd/vhosts/thalasoft.net/subdomains/demo/httpdocs/

I would like to avoid ducplicating the library of php files in these different web sites directories.

I would prefer to keep only one copy of these library php files on the server.

The server is a dedicated server.

The library engine/ is sitting in the directory

/home/httpd/vhosts/thalasoft.net/httpdocs/engine
I use this alias to have the engine/ directory present in the other directories.

Alias /engine "/home/httpd/vhosts/thalasoft.net/httpdocs/engine"

<Directory "/home/httpd/vhosts/thalasoft.net/httpdocs/engine">
Options Indexes MultiViews FollowSymLinks
AllowOverride None
Order allow,deny
Allow from all
</Directory>

But when calling this
http://demo.thalasoft.net/engine/system/admin/login.php
I get a popup window offering to download the php source file.

Whereas calling this http://www.thalasoft.net/engine/system/admin/login.php
works fine and the file is parsed.

Any clue?

Regards

Stephane