php variable scope and fwrite inside function

php variable scope and fwrite inside function

I'm trying to figure out where my add to cart process is messing up, so i
decided to add in some logging code (fwrite) Well then I quickly learned
about php variable scope. And now I'm stuck.
The first thing i tried before i learned about variable scope.
$fp = fopen('logs/functions.txt', 'w');
function addtocart($pid,$qty){
fwrite($fp, 'addtocart()\nProduct ID: '. $pid .'\nQuantity: '. $qty
.'\n');
if($pid<1 or $qty<1) return;
if(is_array($_SESSION['cart'])){
if(product_exists($pid)) return;
$max=count($_SESSION['cart']);
$_SESSION['cart'][$max]['productid']=$pid;
$_SESSION['cart'][$max]['qty']=$qty;
}else{
$_SESSION['cart']=array();
$_SESSION['cart'][0]['productid']=$pid;
$_SESSION['cart'][0]['qty']=$qty;
}
}
fclose($fp);
So that returned an error saying something like fp is not defined.
Then i looked up php variable scope. Because if a similar thing were
written in another, it may have worked.
I tried declaring $fp global;
function addtocart($pid,$qty){
global $fp;
fwrite($fp, 'addtocart()\nProduct ID: '. $pid .'\nQuantity: '.
$qty .'\n');
I get the error "Warning: fwrite(): 3 is not a valid stream resource" like
its turning $fp into some sort of integer. Why?