Using a string starting with @ in a POST

Hello everybody

i'm coding an application which use the twitpic api.
I have to send three postdata $var which are
key = my api key
message = a message (string)
media = @/file/to/upload

Great, but my message is often "@twitteruser here is the photo !"  
(without "), and so curl try to find a file and return a
"failed creating formpost data"
I tried to urlencode and rawurlendode the $message var, it works, but  
twitpic doesn't decode it correctly
Is there a way to pass a string starting with @ whithout having curl try to  
process it as a file ?

(see what happen when using urlencode($message) :
http://twitpic.com/2tngrw

)

Thanks

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