Classes In Php

I have a class and defined as follows:

class theclass
{ var a;
var b;


function seta(x)
{this->a=x;
}

function setb(x)
{this->b=x;
}

}

global $myclass;
$myclass=new theclass;

This is saved in a fileone.php

Then I use it in the fisrt script file as

include_once("fileone.php");
global $myclass;
$myclass->seta(7);
includeonce("filethree.php");

this one is saved as filetwo.php.

My question is on using this class in filethree.php and filefour.php as follows:

Suppose that my filethree.php is

global $myclass;
myclass->setb(8);
$Z=$myclass->a+$myclass->b;

I will get a Fatal error: Call to a member function a on a non-object.

To avoid this error, I would do filefour.php as follows

include_once("fileone.php");
global $myclass;
myclass->setb(b);
$Z=$myclass->a+$myclass->b;

I would avoid a fatal error but would get a less sever error that calling a member value a that has not been set, or something like that althought I know that I set "a" in filetwo.php

Questions:

(1) Since I declared my class as global in all files, I would expect that all script files will see it and I only need to declare it in the first script file that calls it, in this case filetwo.php after which filethree.php would have seen it without having to include-once again.

(2) If I include_once it in filefour.php, does it mean that the member variable set by filetwo.php will be replaced or destroyed?