由于最近用的一个程序需要exif扩展的支持，所以只能试着配置了一下exif扩展。
首先在php.ini里面把;extension=php_exif.dll前面的分号去掉。然后重启IIS，重启之后还是不行，那到底是为什么呢。
经过一番查找，终于在PHP官方网站上找到了原因，官方网站上有这么一段话：
If you have exif.so or php_exif.dll enabled under php.ini dynamic extensions, but you still have problem running any funtion, it is possible that you may need to change order of extensions in php.ini.
In many PHP installation packages the extensions are listed alphabetically. However, exif is dependent on mbstrings to be loaded first. Yet, with alphabetic ordering the exif is listed before mbstrings and it causes the error. Load mbstrings extensions before exif.
Thus, do put exif as the very last extension to load and you should be fine.
大概意思是说exif扩展需要mbstrings扩展支持，由于大部分的php.ini文件里面exif扩展都在mbstrings扩展前面加载，所以这时候的exif找不到mbstrings就出错了。