PHP Calling dynamic functions

Im trying to figure out how to call functions based on what a user clicks on a form. But im not sure if im doing it right.

I have a number of classes, lets say 3 for different ways to connect to a site, the user clicks on which one they would like.

1. FTP
2. SFTP
3. SSH

Which i have named 'service' in my code.

I don't want to run a whole bunch of IF statements, i would rather try and build the call dynamically.

What i have at the moment is as follows

$ftp_backup = new FTPBackup;
$sftp_backup = new SFTPBackup;
$ssh_backup = new SSHBackup;

$service = $request->input('service') . '_backup';
$service->testConn($request);

Im getting the following error

Call to a member function testConn() on string

Im not sure im doing this right.

Any help would be greatly appreciated.

Thanks

via Chebli Mohamed