functions file, plugin, and theme scoping in wordpress mu

I've got a plugin that is declared and hooked following best practices described in this related question:

Wordpress: Accessing A Plugin's Function From A Theme

So it looks (platonically) like this:

if ( !class_exists( 'Foo' ) ) {
  class Foo {
    ...
    public function do_stuff() {
      // does stuff
    }
  }
}

if ( class_exists( 'Foo' ) ) {
  $MyFoo = new Foo();
}

Now, if I call $MyFoo->do_stuff() from a theme file, such as, say, single.php, $MyFoo in fact does_stuff and I see the output in the page.

However, if I write a function in functions.php that wants to call $MyFoo->do_stuff() and then call that function from single.php the object is not found. In summary,

Works:

in themes/my_theme/single.php:
  if (isset($MyFoo))
    $MyFoo->do_stuff();

Does not work:

in themes/my_theme/functions.php:
  function do_some_foo_stuff() {
    ...
    if (isset($MyFoo)) {
       $MyFoo->do_stuff();
    } else {
       echo "no MyFoo set";
    }
    ...
  }
 themes/my_theme/single.php:
   if (isset($MyFoo))
     do_some_foo_stuff();

Outputs -> "no MyFoo set"

This may be totally unsurprising, but it's something I need/want to work, so if anyone can explain what is going on it'd be appreciated. Why can't the theme's functions file (or other 开发者_JAVA百科plugin files in mu-plugins for that matter) find $MyFoo object?

Read up on variable scope. The variable $MyFoo is not accessible within the function do_some_foo_stuff() unless you declare it global first;

function do_some_foo_stuff()
{
    global $MyFoo;
    ...
}