t1.php

<?php

namespace A {

	class ClassA {

		public function doA()
		{
			try 
			{
				try 
				{
					$b = new \B\ClassB();
				}
				
				// Add a \ to this one to see it catching the exception at this point
				catch(Exception $e)
				{
					echo "(1) catched exception without root namespace";
				}
			}
			
			// Exception may not be catched by the first one, but will by this one
			catch(\Exception $e)
			{
				echo "(2) catched exception with root namespace";
			}
		}

	}

}

namespace B {

	class ClassB {

		public function __construct()
		{
			throw new \Exception("this is an exceptio!", 1);
		}

	}

}

t2.php

<?php

require "t1.php";

$a = new A\ClassA();

$a->doA();