updating a single file

Hi all,
I'm trying to control, via a php web page, the updates of a working copy.
Updating an existing file is quite easy : 'svn up path/to/file'. But how
can I update a file wich was added to the repository after last update?
Obviously, svn up path/to/file doesn't work, because 'file' isn't present
in the working copy. I tried 'svn co full_URL_of_file
path/to/workingcopy', and it didn't work either (message is : 'svn: '' is
already a working copy for a different URL').
I also tried 'svn cat $1 > $2', but the problem is that it creates a new
file which isn't under version control, and 'svn up $2' won't overwrite
it.
Is there any way I could copy this file into my working copy, and include
it under version control?
Thanx in advance.

-- 
Erwan Lacoste
Stagiaire
RHAPSO
1, rue Galvani
91300 Massy
Tel +33 (0)1 60 13 79 50
Fax +33 (0)1 69 32 19 61
---------------------------------------------------------------------
To unsubscribe, e-mail: users-unsubscribe@subversion.tigris.org
For additional commands, e-mail: users-help@subversion.tigris.org