It seems like this is a problem with the permissions. If it's Unix or Linux and an Apache Webserver, the Webserver is running usually as user id nobody and group id nobody.
In this case chmod 755 won't give you enough permissions to write your file in to the corresponding directory if your personal user id and group id is set.
If this is the case, chmod your directory to 777. Then you should basicly be able to copy your file.
Maybe aswell have a look at:
move_uploaded_file ( string filename, string destination)
Cheers
Chris