很好用的DB类，结构清晰简洁。自己编写的，希望再进行改进改进。只做参考，没有性能上的优化。-DB use a very good category, the structure of clear and concise. I have written, I hope to improve further improvement. Reference only, not to optimize performance.

PDO exteneds my db cla-PDO exteneds my db class