I'm just a beginner in PHP coding. I've been reading through a tutorial, but having some trouble with basic PHP concepts. If you could help me, I'd be much obliged.

I'm having trouble understanding why the following code doesn't work.

```php
<?php
function sum($x, $y) {
    $z = $x + $y;
    return $z;
}

echo "$x + $y = " . sum(5,10);
?>

```

When I run it, it prints `Array + Array = 15`, but I was aiming for `5 + 10 = 15`. 

Could someone please explain to me why is prints `Array` instead of the actual values?

Thanks! :)

---

Sorry about the typos and errors in the code.
Corrected it, but still trying to figure out why I cannot declare `$x` and `$y` within `sum(5,10)`.