main1.php

include "MyStuff_MyClass.php";
include "MyClass.php";

new MyStuff\MyClass(); // prints "mine";
new \MyClass(); // prints "global";
new MyClass(); // Prints "global";

main2.php

include "MyStuff_MyClass.php";
include "MyClass.php";

use MyStuff;

new MyStuff\MyClass(); // prints "mine";
new \MyClass(); // prints "global";
new MyClass(); // Prints "mine";

MyClass.php

class MyClass {
   function __construct() { echo "global"; }
}

MyStuff_MyClass.php

<?php

namespace MyStuff {
   class MyClass {
      function __construct() { echo "mine"; }
   }

}


new MyClass(); // prints "global";
new MyStuff\MyClass(); // prints "mine";