php进行动态访问时命名空间如何使用（代码）

<?php
namespace {

const con=0;
const test="test";
function foo(){
    echo &#39;foo in global&#39;."\n";
}
class A{
    static function foo(){
        echo &#39;foo in global\A&#39;."\n";
    }
}
}
namespace a\b{
const con = 1;
const test="a\b\\test";
function foo(){
    echo &#39;foo in a\b\foo&#39;."\n";
}
class A{
    static function foo(){
        echo &#39;foo in a\b\A&#39;."\n";
    }
}


$a=&#39;A&#39;;
$obj=new $a();
$obj->foo();
$a=&#39;\A&#39;;
$obj=new $a();
$obj->foo();
$a=&#39;a\b\A&#39;;
$obj=new $a();
$obj->foo();
$a=&#39;\a\b\A&#39;;
$obj=new $a();
$obj->foo();

$b="con";
echo constant($b)."\n";
$b="\con";
echo constant($b)."\n";
$b="a\b\con";
echo constant($b)."\n";
$b="\a\b\con";
echo constant($b)."\n";

$c="foo";
$c();
$c="\\foo";
$c();
$c="a\b\\foo";
$c();
$c="\\a\\b\\foo";
$c();
}
?>