ssh2_disconnect(...) requires at least PHP 7 (as it requires PECL ssh2 >= 1.0, which the PECL documentation on https://pecl.php.net/package/ssh2 states: "ssh2 1.0 and above is PHP 7 only").
For PHP 5.x similar functionality, try setting the ssh2_connect session variable to null, or unsetting it completely - this should allow PHP's garbage collection close the connection.
Example:
$session=ssh2_connect(...); // open connection
// code here to use connection
$session = null; unset($session); // close connection