preferentially use SFTPv3, since that's waaaay more tested than any other version

phpseclib/Net/SFTP.php

@@ -205,6 +205,15 @@ class Net_SFTP extends Net_SSH2
      */
     var $defaultVersion;
 
+    /**
+     * Preferred SFTP version
+     *
+     * @var int
+     * @see self::_initChannel()
+     * @access private
+     */
+    var $preferredVersion = 3;
+
     /**
      * Current working directory
      *