How to create and use a library of functions

      <php
      // Add an item to the cart 
      function cart_add_item(&$cart, $name, $cost, $quantity) {
        $total = $cost * $quantity; 
        $item = array(
          $name, 'cost' => $cost, 'qty' => $quantity, 'total' => $total 
          ); 
        $cart[] = $item; 
      } 
      // Update an item in the cart 
      function cart_update_item(&$cart, $key, $quantity) { 
        if (isset($cart[$key])) { 
          if ($quantity <= 0) { 
            unset($cart[$key]);
          } else { 
            $cart[$key]['qty'] = $quantity; 
            $total = $cart[$key]['cost'] * $cart[$key]['qty']; 
            $cart[$key]['total'] = $total; 
          } 
        } 
      }
      ?>
      

      // load the library 
      require_once('cart.php'); 
      // create an array to store the cart 
      $cart = array(); 
      // call methods from the library 
      cart_add_item($cart, 'Flute', 149.95, 1); 
      cart_update_item($cart, 0, 2);  // update the first item (key of 0)
      $subtotal = cart_get_subtotal($cart); 
      // display the result 
      echo("<p>This subtotal is $" .$subtotal . "</p>\n");
      

I generally name my library of functions either functions.php, dbfunctions.php, and formfunctions.php.