全局变量和局部变量的内存地址

Question

I've just used local and global variable with the same name. Local variable located in a function. The code snippet is as below (in PHP):

$var = 10;
function fn ()
{
    $var = 20;
    return $var;
}
fn ();
echo $var;

If the global variable contain 10 then after calling the fn() how variable $var remain unchanged where $var is assigned to 20 in the function. Both have same name, my question is how memory track which one is global and which one is local?