I'm stuck at functions

and $result stores the returned value from the function. 

Yes and all your comments are correct. When I set the var $sum to zero is just for ensure that the count starts at zero and have a correct sum of the numbers.

<?php

// Set a variable $numbers with the array
$numbers = array(1,2,3,4);

// Count the total number of numbers and put it in the variable $total
$total = count($numbers);

// Set the variable sum to zero. Q: Why do we need to se this to zero? Is this because it needs to be a numeric?
$sum = 0;

// Set the output variable to empty. Q: Why does it need to be empty and not 0? Or 4?
$output = "";

// Variable i to 0. Why?
$i = 0;

// Every individual array item is put in an $number variable
foreach($numbers as $number) {

// Variable $i counting, adding one for every $number variable added
    $i = $i + 1;

// If the $i variable is less than the total (4) add it to the $number variable. When the variable $i hits 4 it stops.
    if ($i < $total) {

        $sum = $sum + $number;

    }

}

// Echo's the $sum variable that contains the result of the $numbers total
echo $sum;

?>

It means that the function needs two values, in other case if the parentheses are empty we don't need special values (arguments).

```<?php
function sum_array($array = []){
     // Return 0 if the argument is not an array or if the 
     // array is empty
     if(!is_array($array) || count($array) == 0) return 0;

     $sum = 0;
     foreach($array as $number){
         // Short notation for sum = sum + number;
         // and ensure that $number is always a number with (int)
         $sum += (int)$number;
     }

     return $sum;
 }

 // usage
 $numbers = array(1,2,3,4,5,6,7,8,9,10);
 // call to the function and save the value to the var $sum
 // where $sum is different from the variable $sum of the inside of the function
 $sum = sum_array($numbers);
 // $sum should be 55
 echo $sum;