This is where I seem to get confused. There are so many elements/names, can someone explain $array as $element?

Any and all variables can be called anything (though there are a couple of rules that apply). In this case it's called $array just to make it obvious that it's an array. But you could call it $jerry if you prefer :D

In your example, $array is only seen in the function. The important thing about function is that any and all code within the function is almost like a bubble -- abstract from the rest of the code. For example, you cannot use a variable that you used ealier in the code, in contrast to "regular code". You must see functions and the "main code" as two separate and independent pieces of code that do not interract with eachother. The only common they have is that they are written in the same document.

If you want to send a variable (or as many as you want) you must send what's called a parameter to the function. Parameters are denoted by the values stated within the parenthesis next to the function name. In your example, your function is called mimic_array_sum and accepts one variable which it calls $array.

What's important to note here that whenever you call the function mimic_array_sum and pass it an argument, the function will still call that argument $array. When you pass an argument you essentially make the function make a copy of that variable. It then does the operations that are written in that function, and when done returns a result back to the "main code".

Here is a code example:

function find_double($nbr) {
    $result = 2 * $nbr;
    return $result;
}

$nbr = 5;
$variable = 9;

$test1 = find_double($nbr);
$test2 = find_double($variable);
$test3 = find_double(13);

echo $test1;
echo $test2;
echo $test3;

First, I declare a function which I call find_double. This function accepts one variable, which locally (and only locally within the function) is called $nbr. Then I make two variables with values 5 and 9. Finally I call the function 3 times, each time giving it a different parameter. The $test1/2/3 will store the return-value of the function which I then echo out.

As you can see, when I call the function, I can pass any variable or value I want. Name doesn't matter. It can be the same name as used locally inside the function, or not. The function will be run 3 times because it's called 3 times. If we were to somehow pause the function midway and look up the value of $nbr, we would see that it has the value: 5, the first time it's called, then 9 and finally 13.

So to answer your question, $array was never created because it's a parameter for a function -- it will just copy over whatever value you sent into it. So if you want to send it an array, you probably have created it elsewhere in your code. Indeed, it was created on the line:

$palindromic_primes = array(11, 757, 16361);

Here you create an array called "palindromic_primes" with the 3 values stored within it: 11, 757, and 16361. On the next line you send this array into the mimic_array_sum function. The function will return a result at some point which the "main program" awaits to retrieve and store in the variable $sum.

If you haven't already, check out Treehouses PHP function course. It's quite small and easy to do within a day or two: http://teamtreehouse.com/library/php-functions-2