I tested your example and it looks like it works if you supply -d
option to unzip
command and specify the directory where the file should be unpacked:
if (!($stream = ssh2_exec($con, 'unzip /home/servers2/'.$mUsername.'/Tekkit_Server_3.1.2.zip -d /home/servers2/'.$mUsername))){
echo "fail: unable to execute command\n";
}