how can access namespace of sub-namespce in php

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PHP

hi all,

I am learning of php, I don't know how can access namespace of sub-namespace?

Could you suggest me?

my code is:

<?php

namespace food
{
	class Cat
	{
		function says() 
		{
			echo 'Cat';
		}  
	}
}
?>
<?php
namespace good
{
	class Rat
	{
		function says() 
		{
			echo 'Rat';
		}  
	}
}

?> 

<?php

use food as foo;

$objs=new foo\Cat();

?>

could you tell me where I went wrong?

thanks in advance...................