Working with Functions

Here is the code in my products.php file

<?php //line 4 use concatenation to build list item as a piece of text, create output with a blank value function get_list_view_html($product_id, $product) {

        $output = $output . "<li>";
        $output = $output . '<a href="shirt.php?id=' . $product_id . '">';
        $output = $output . '<img src="'. $product["img"] .'" alt="'. $product["name"] .'">';
        $output = $output . "<p>View Details</p>";
        $output = $output . "</a>"; 
        $output = $output . "</li>";
return $output;

}

$products = array(); $products[101] = array( "name" => "Logo Shirt, Red", "img" => "img/shirts/shirt-101.jpg", "price" => 18, "paypal" => "9P7DLECFD4LKE", "sizes" => array("Small", "Medium","large", "X-Large") ); $products[102] = array( "name" => "Mike the Frog Shirt, Black", "img" => "img/shirts/shirt-102.jpg", "price" => 20, "paypal" => "SXKPTHN2EES3J", "sizes" => array("Small", "Medium","large", "X-Large") ); $products[103] = array( "name" => "Mike the Frog Shirt, Blue", "img" => "img/shirts/shirt-103.jpg",
"price" => 20, "sizes" => array("Small", "Medium","large", "X-Large") ); $products[104] = array( "name" => "Logo Shirt, Green", "img" => "img/shirts/shirt-104.jpg",
"price" => 18, "sizes" => array("Small", "Medium","large", "X-Large") ); $products[105] = array( "name" => "Mike the Frog Shirt, Yellow", "img" => "img/shirts/shirt-105.jpg",
"price" => 25,

); $products[106] = array( "name" => "Logo Shirt, Gray", "img" => "img/shirts/shirt-106.jpg",
"price" => 20 ); $products[107] = array( "name" => "Logo Shirt, Turquoise", "img" => "img/shirts/shirt-107.jpg",
"price" => 20 ); $products[108] = array( "name" => "Logo Shirt, Orange", "img" => "img/shirts/shirt-108.jpg",
"price" => 25 );

?>

When I click on http://localhost/shirts.php I get the following error: Notice: Undefined variable: output in C:\xampp\htdocs\inc\products.php on line 5

I do not know how to correct this error; I do not understand the purpose of the code : [$output = $output . "<li>";]