引用该文件……

Manual Installation Steps

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Upgrading from a prevIoUs PHP version: PrevIoUs editions of the
manual suggest moving varIoUs ini and DLL files into your SYSTEM
(i.e. C:\WINDOWS) folder and while this simplifies the installation
procedure it makes upgrading difficult. We advise you remove all of
these files (like PHP.ini and PHP related DLLs from the Windows
SYSTEM folder) before moving on with a new PHP installation. Be sure
to backup these files as you might break the entire system. The old
PHP.ini might be useful in setting up the new PHP as well. And as
you’ll soon learn,the preferred method for installing PHP is to
keep all PHP related files in one directory and have this directory
available to your systems PATH.

有了这个警告,应该只是在新文件夹中安装新版本的PHP并更新IIS配置以使用新路径.