$cmds = array(
    // this proves that the basic call works
    'ssh martin@localhost echo Hallo',

    // this does not hang
    'ssh martin@localhost tar cf /tmp/try.tgz /root/no/files',

    // this hangs here!
    'ssh martin@localhost tar cf /tmp/try.tgz /root',

    );

foreach ($cmds as $cmd) {
    echo "\nCMD:".$cmd; flush();
    echo "\nPASSTHRU output:\n";
    passthru($cmd);
    echo "\n\nPROC_OPEN started...\n"; flush();

    $p = proc_open($cmd,
                   array(0=>array('pipe','r'), 1=>array('pipe','w'), 2=>array('pipe','w')),
                   $pipes);
    echo "\nSTDOUT output:\n".stream_get_contents($pipes[1]);
    echo "\nSTDERR output:\n".stream_get_contents($pipes[2])."--------------\n\n\n";
}