the FTP and file upload functions are well documented in the manual: http://www.phpbuilder.com/manual/ref.ftp.php and http://www.php.net/manual/en/features.file-upload.php .
if you read both of these and are still having trouble, please provide more info on the specific problem you are having. (be especially mindful of file permissions on the directory you are writing to.)
in other words, what does "my host doesn't allow file uploads via PHP" mean exactly? you have verified you can accept POST data from an "input type='file'" element, but you cannot use php's file writing functions to save it?
if you really have to go the ftp route, the only tricky part (besides the whole lotta lag in establishing a ftp connection) will be finding which directory holds the temporary files on the system ... try /tmp .