最近在PHP官网上下载了PHP中英文chm格式手册各一份 (http://www.php.net/download-docs.php)，但是下完发现打开只能看到导航，内容却是看不到的。一直以为是下载的chm文件有问题，重复下载，重复出现这样的状况。

后来，我在下载页面看到了这样的提示：

If you are using Windows XP SP2 or later and you are going to download the documentation in CHM format, you need to "unblock" the file after downloading it by right-clicking on it in Windows Explorer and selecting the "Properties" menu item, then clicking on the "Unblock" button (on Windows Vista this is within the "Security" options). Failure to unblock the documentation file may result in error messages including "Navigation to the webpage was canceled" due to Windows security restrictions.

这才恍然大悟。

上面说的是：

如果你使用Windows XP SP2 或者更高的版本，你需要在下载之后通过右键(Right-clicking)->属性(Properties)，然后点击解锁(”Unblock”)。在Windows Vista里可能是安全(“Security”)。因为Windows的安全限制，如果解锁失败的话，可能会有一个错误提示：导航到网页已被取消(”Navigation to the webpage was canceled”) 。