How to use Web pages to determine whether the software is installed on the user's machine

How to use the Web page to determine whether the software is installed on the user's machine
We have developed a small software for the Web site, has its own information chat features, including reading the site information.
Now I would like to ask, how to use the Web page to determine whether the user has installed this software on the machine? If installed, you can start the message window for this software, if not, go to the Download installation page.

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------Solution--------------------
I have asked this question once.

I was told that it was against the law. -_-!!!

Later I asked the manager. Because we were doing this stuff.

The manager told me that it was the software side that provided the plugin when the software was installed.

ActiveX plugin.

Then the Web page can be used to determine the JS code has not installed this plugin.

This will tell you if you have the software installed on your computer. Well..
------Solution--------------------
PHP is not.

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