ob_clean这个函数的作用就是用来丢弃输出缓冲区中的内容，如果你的网站有许多生成的图片类文件，那么想要访问正确，就要经常清除缓冲区。
if you work on an extremely large project with a lot of source and required files, like myself, you will be well-advised to always clear the output buffer prior to creating an image in php.