Please explain step by step the get_list_view_html function and how it works in conjunction with the foreach loop

Ok, so:

<?php 
        foreach($products as $product_id => $product) { 
        echo get_list_view_html($product_id,$product);
        }
?>

loops through all the products in the $products array one at a time and assigns it to two working/temporary variables, $product_id and $product. That's what this does:

as $product_id => $product

The "as" keyword tells PHP to assign the current index of the array to the $product_id, and the "=>" tell's PHP to assign the array stored at the current index to $product. When I write loops like this I always think:

foreach( $item_in_an_array as $item_index => $item_value)

That way I don't confuse things, because you can also write a foreach loop like this:

foreach( $item_in_an_array as $item_value)

if you don't really care what the $value's position in the array is.

Now, in programming, doesn't really matter the language, the code inside the loop is executed every time the loop runs. So if you are loop through an array with ten items, the code inside the loop will run ten times. In this case, the code inside the loop is:

echo get_list_view_html($product_id,$product);

So every time the loop runs it echoes the value of the get_list_view_html() function. It's important to note that computers read code from left to right, but execute code from right to left. This means that get_list_view_html is executed before echo is executed.

The get_list_view_html() function takes two arguments, $product_id and $product, which correspond to the working variables the foreach loop uses as temporarily store the index and value of the current item in the loop. When these values are passed inside of the function, they are used to generate the html to display each product.

Now for the "why and how" of the functions arguments. The "why" of adding arguments to a function is to ensure that data needed inside the function will be available to the function. PHP does this by throwing a fatal error if a function defined with arguments:

function cat_in_the_hat( $thing1, $thing2){
    // some code that causes mayhem.
}

is called without any arguments supplied:

// no mayhem will be caused because
// cat_in_the_hat() doesn't have $thing1 and $thing2
cat_in_the_hat();

In regards to the product code, Randy decided to make sure that product information had to be given to the function so that it could generate the html for the product.

The "how" arguments work in a function is similar to the way arguments in a foreach loop work. When a function is defined with arguments:

function get_list_view_html($product_id, $product) {
    // code to generate html
}

those arguments are used as temporary variables when the function is executed. In this case, the arguments are used to temporarily hold the $product_id and $product info while being used inside the function to generate parts of the html like the link's href:

'<a href="shirt.php?id=' . $product_id . '">

Hopefully, this will clear some things up for you. If you need anything else cleared up just let me know.