conditionaly including classes/functions

Is it okay practice to condtionally include php files that contain only classes or functions (as opposed to just straight code)? The result is a class or function inserted right in the middle of an if{} block: } elseif ($var=='a') {

    include('temp.php');

    $obj = new foo;
    echo $obj->hello();

} The above code expanded looks like this (below). Works, but it seems wierd to be able to have a class in the middle of an if{} code block. } elseif ($var=='a')

    class foo
    {

function hello() {

  	    return "Hello";

}

    }

    $obj = new foo;
    echo $obj->hello();

} Andrew