Meaning of :: Operator in C# and PhP

using MyNameSpace;

...

global::System.Data

object->method();
object->property;

class::method();
class::property;

object.method();
class.property;

<?php
class Circle {
    public $radius;
    public $centerX;
    public $centerY;
    public static $PI = 3.1415;
    public function __construct($r,$x,$y) {
        $this->radius = $r;
        $this->centerX = $x;
        $this->centerY = $y;
    }
    public function getArea() {
        return self::$PI * $this->radius * $this->radius;
    }
}
$circle1 = new Circle(10,0,0);
echo $circle1->getArea() . "<br>n";
$circle2 = new Circle(15,10,12);
echo $circle2->getArea() . "<br>n";
Circle::$PI = 3.14;
echo $circle1->getArea() . "<br>n";
echo $circle2->getArea() . "<br>n";
?>

namespace Foo {
  namespace System {
    class DateTime {}
    class Example {
      object Example() {
        return System.DateTime.Now; // Returns Foo.System.DateTime.Now
        return global::System.DateTime.Now; // Returns mscorlib,System.DateTime.Now
      }
    }
  }
}

using IOAlias = System.IO;

IOAlias::StreamReader sr = new StreamReader(path);

global::System.IO.StreamReader sr = new StreamReader(path);

public class MyClass { }

namespace namespaceB
{
    public class A
    {
        public class MyClass { }
    }

    public class OtherClass
    {
        public A.MyClass MyAClass;
    }
}

 Explanation
  In the example above, the compiler would always resolve the
  MyAClass to the type namespaceB.A.MyClass.
 If you wanted it to be A.MyClass, there   was no way to do it until C# 2.0.
 In C# 2.0, you would use: public class MyOtherClass
 { public global::A.MyClass yAClass; }
To indicate that you shouldn't use the local namespace scope,
but rather, the root namespace.