/**
     * 中間加密 字串擷取法
     */
    public static function encryptTel($tel) {
        $new_tel = substr($tel, 0, 3).'****'.substr($tel, 7);
        return $new_tel;
    }

    /**
     * 中間加密 替換字串的子串
     */
    public static function encryptTel($tel) {
        $new_tel = substr_replace($tel, '****', 3, 4);
        return $new_tel;
    }

    /**
     * 中間加密 用正則
     */
    public static function encryptTel($tel) {
        $new_tel = preg_replace('/(\d{3})\d{4}(\d{4})/', '$1****$2', $tel);
        return $new_tel;
    }
 

很實用。