/**
 * Switch TOR to a new identity.
 **/
function tor_new_identity($tor_ip='127.0.0.1', $control_port='9051', $auth_code=''){
    $fp = fsockopen($tor_ip, $control_port, $errno, $errstr, 30);
    if (!$fp) return false; //can't connect to the control port

    fputs($fp, "AUTHENTICATE \"$auth_code\"\r\n");
    $response = fread($fp, 1024);
    list($code, $text) = explode(' ', $response, 2);
    if ($code != '250') return false; //authentication failed

    //send the request to for new identity
    fputs($fp, "signal NEWNYM\r\n");
    $response = fread($fp, 1024);
    list($code, $text) = explode(' ', $response, 2);
    if ($code != '250') return false; //signal failed

    fclose($fp);
    return true;
}

/**
 * Load the TOR's "magic cookie" from a file and encode it in hexadecimal.
 **/
function tor_get_cookie($filename){
    $cookie = file_get_contents($filename);
    //convert the cookie to hexadecimal
    $hex = '';
    for ($i=0;$i<strlen($cookie);$i++){
        $h = dechex(ord($cookie[$i]));
        $hex .= str_pad($h, 2, '0', STR_PAD_LEFT);
    }
    return strtoupper($hex);
}

TOR Authentication

Depending on your configuration, the script may need to authenticate before it can signal TOR to switch to a new identity.

There are three authentication modes TOR supports :

• None – no authentication required. You don’t need to set the third parameter of tor_new_identity().
• Cookie – TOR stores a “magic cookie” in the control_auth_cookie file inside it’s data directory. You have to authenticate by sending this cookie encoded in hexadecimal form. You can use tor_get_cookie() to read and properly encode the cookie.
  Example :
  

  $cookie = tor_get_cookie('/path/to/tor/data/dir/control_auth_cookie');
  tor_new_identity('127.0.0.1', 9051, $cookie);

• Password – your basic password authentication. To authenticate you will need to use the password in “quotes” or a hex-encoded version of the password.
  Example :
  

  tor_new_identity('127.0.0.1', '9051', '"secret_passw0rd"');

http://w-shadow.com/blog/2008/06/20/tor-how-to-new-identity-with-php/