When installing the x64 version on x64 OS, PowerShell snapin is not registered, so Add-PsSnapin PHPManagerSnapin will not found it, because the DLL is not registered.

Cause:

Installutil probably did not run during the setup. It is not in the path, so I am not sure how should the installer find it. Anyway the plugin is not registere neither in the normal neither in the WOW64 hive.

Workaround

Important: Must run the 64 bit version of installutil. For this we must find the intallutil... for example:

"C:\Windows\Microsoft.NET\Framework64\v4.0.30319\InstallUtil.exe" "C:\Program Files\PHP Manager 2 for IIS\Web.Management.PHP.PowerShell.dll"