If you get

Call to undefined function hash_equals() error

even if your patch was successful then it may mean that you have copied function.php in app/code/local/Mage/Core.

You will have to insert that function there too because that file overwrites the core one.

If you get

Call to undefined function hash_equals() error

even if your patch was successful then it may mean that you have copied functions.php in app/code/local/Mage/Core.

You will have to insert that function there too because that file overwrites the core one.

So insert in app/code/local/Mage/Core/functions.php at the end:

if (!function_exists('hash_equals')) {
    /**
     * Compares two strings using the same time whether they're equal or not.
     * A difference in length will leak
     *
     * @param string $known_string
     * @param string $user_string
     * @return boolean Returns true when the two strings are equal, false otherwise.
     */
    function hash_equals($known_string, $user_string)
    {
        $result = 0;

        if (!is_string($known_string)) {
            trigger_error("hash_equals(): Expected known_string to be a string", E_USER_WARNING);
            return false;
        }

        if (!is_string($user_string)) {
            trigger_error("hash_equals(): Expected user_string to be a string", E_USER_WARNING);
            return false;
        }

        if (strlen($known_string) != strlen($user_string)) {
            return false;
        }

        for ($i = 0; $i < strlen($known_string); $i++) {
            $result |= (ord($known_string[$i]) ^ ord($user_string[$i]));
        }

        return 0 === $result;
    }
}