“EnyimMemcached” need .NET Framework 3.5 or later version to use. Please refer to here for details: https://github.com/enyim/EnyimMemcached. If you install “EnyimMemcached” to any projects which target to previous version of .NET Framework that before 3.5, you will get the same error.
Since PHP proejct isn’t a .NET Framework type project, you could not install “EnyimMemcached” to a PHP project. If you want to use “memcached” in a PHP project, you need to install and configuration based on below document that from PHP site.