Your PHP example is possible to write in a way that is nearly identical to the C# you provided. Why are you prefixing Namespace\MyClass(); when you declared your namespace scope in 'use \My\Namespace'? Your assignment can just be:

  $var = new MyClass();

This brings it pretty much in line with your C# example. Have you read the PHP docs on namespaces?

http://www.php.net/manual/en/language.namespaces.basics.php