以下是我的脚本：

    <?php
    $connection = ssh2_connect('XX.XX.XX.XX', 22);
    ssh2_auth_password($connection, 'root', '******');

    $stream = ssh2_exec($connection, 'useradd -d /home/users/test -m testftp');
    $stream = ssh2_exec($connection, 'passwd testftp');
    $stream = ssh2_exec($connection, 'password');
    $stream = ssh2_exec($connection, 'password');
    ?>

它显示以下错误：

Fatal error: Call to undefined function ssh2_connect() in /home/chaosnz/public_html/fotosnap.net/test.php on line 2

我该如何处理？

谢谢