Whoever told you that is confused with something else.
Browsers do not expose that kind of hardware information to a web server at any point, so there is no existing mechanism for a server-side PHP script to get a visitor's HWID. JavaScript provides very limited information about display resolution and color depth, but certainly not something like HWID. A very long time ago, it used to be possible to use ActiveX objects in JavaScript in IE to retrieve hardware information via WMI, but you can bet that's not possible anymore, especially on a non-intranet zone site. You'd most certainly need some kind of browser plugin or extension; Flash does not provide the HWID, so you're looking at something custom-written by you that must be installed on a visitor's machine either prior to or during their visit.
You're better off continuing to sell the app that gets the HWID itself and then accesses your web site, because that's going to be far less painful than writing browser extensions and plug-ins for all major browsers to provide that information in a secure way.