Before OO came along, functions were all we had!

We RPG developers are still relatively new to the concept of functions. I say "relatively" because some of my readers are still under the impression that embedded SQL is a "new" feature. Perhaps we can just agree that "new" is in the eye of the beholder and leave it at that. But I digress. This article discusses functions in PHP: how to find them, how to write them, and why they are useful.

The Basics

In RPG, you could even today write a program without using any subprocedures or functions (subprocedures that return a value are really functions). This would, hopefully, be a very simple program, and therefore you wouldn't have need of any. In PHP, this is not really the case. In all but the very simplest PHP script (anything beyond "hello world.") you will be using functions. Part of this is due to the fact that the PHP language is made up of functions; before OO came along, functions were all we had. Take the following code, for instance.

<?php

$currentTime = date(r);

print_r($currentTime);

echo "<br>";

?>

This code will display the date and time formatted very nicely. At first glance, you might not think that you are using a function in this script, but you are. Print_r is a function that displays information about a variable in a format that is easy for us to read. It can even return a value. Let's change the function call a bit to demonstrate.

<?php

$currentTime = date(r);

$displayTime = print_r($currentTime, TRUE);

echo $displayTime."<br>";

?>

This modified code will output exactly the same thing, but it demonstrates how the print_r function can return a value.

PHP supports about 5,000 functions. You most likely will not use an appreciable percentage of all those functions, but it's worth looking for a ready-made function in the PHP documentation before setting off to write your own. Some of the functions that you may already be familiar with are these:

• db2_connect

• mysql_connect
• trim
• substr
• print_r
• and many others

On a related note, "print" and "echo" are what is known as language constructs; they are not functions and do not return a value.

In addition to those functions supported and developed in the PHP language itself, there are hundreds (if not thousands) more in PHP Extensions. Some of these can be found at http://www.phpclasses.org and http://pear.php.net. These Web sites are repositories for open-source extensions for PHP. What that means is that the classes/functions that are available have been developed and (usually) tested rigorously before being made available to you as the user. Best of all, you can use them in any application without paying licensing or fees of any sort. A couple of my favorite extensions are the Pear mail and mail_mime functions (see "TechTip: Email Made Simple(r) with PHP and Pear").

User-Defined Functions

The real power of functions becomes apparent when you start writing your own. Similar to using a service program to access commonly used functions in RPG, writing your own functions in PHP saves time and makes for more maintainable and reusable code.

When you define your own functions, there some rules you must be aware of (see www.php.net for more details):

1. You must give it a name (obviously!).
2. Functions always return a value, even if you don't specify it. By default a function will return a null value.
3. Function variables have local scope.

Let's create a function to see what I mean. Our sample function will determine volume discounts given to customers. We will give a 15 percent discount on orders of more than 10 items and 20 percent on orders of more than 50 items. Orders of less than 10 items will receive no discount.

<?php

function get_volume_discount( $original_price, $quantity="zero") {

switch( $quantity) {

      case  ($quantity == "zero"):

            $discount = 0;

            break;

      case   ($quantity > 50):

            $discount = .2;

            break;

      case   ($quantity > 10):

            $discount = .15;

            break;

      default:

            $discount = 0;

}

$discounted_price = $original_price * ( 1 - $discount );

return $discounted_price;

}

// usage cases

$price = 25.00;

$ordered_items = 5;

$disc_price1 = get_volume_discount( $price, $ordered_items);

echo "Price: ".$price."  Qty Ordered: ".$ordered_items."<br>";

echo "Discounted Price: ".$disc_price1."<br><br>";

$ordered_items = 12;

$disc_price2 = get_volume_discount( $price, $ordered_items);

echo "Price2: ".$price."  Qty Ordered 2: ".$ordered_items."<br>";

echo "Discounted Price2: ".$disc_price2."<br><br>";

$ordered_items = 51;

$disc_price3 = get_volume_discount( $price, $ordered_items);

echo "Price: ".$price."  Qty Ordered 3: ".$ordered_items."<br>";

echo "Discounted Price3: ".$disc_price3."<br><br>";

?>

There are several things to notice here. First, the $quantity parameter is being defaulted to "zero" if the calling program/script doesn't pass it. This is the syntax ( parameter = defaultValue) you would use anytime you wanted to default a value passed to a function. Second, you probably noticed in the SWITCH statement that I check the ordered quantities in descending order. This is because (similar to RPG) PHP will search the associated CASE values in the order they are coded until it finds one that is true. If the $quantity > 10 comparison was before $quantity > 50, then we would never give the 20 percent discount because we could never reach that code. Third, we are returning the discounted price to the calling program, but is there another way we could have achieved this?

Kick It up a Notch

Of course there is. Let's make a few quick modifications to demonstrate some of the flexibility of PHP. Modify your original function to read as follows:

function get_volume_discount( &$original_price, $quantity="zero") {

switch( $quantity) {

      case  ($quantity == "zero"):

            $discount = 0;

            break;

      case   ($quantity > 50):

            $discount = .2;

            break;

      case   ($quantity > 10):

            $discount = .15;

            break;

      default:

            $discount = 0;

}

if ($discount > 0) {

       $original_price = $original_price * ( 1 - $discount );

       return true;

} else {

      return false;

}

}

What have we done? Well, we changed the $original_price parameter to be passed by reference by adding the "&" in front of the variable name. This means that the changes we make to the value $original_price in our function will affect the passed value in the calling script/program. All parameters in PHP functions are passed by value by default. So our original function has the parameters passed by value. See the example below using our new changes.

// usage cases

$price = 25.00;

$ordered_items = 5;

// after this executes $price = 25.00

get_volume_discount( $price, $ordered_items);

echo "Price: ".$price."  Qty Ordered: ".$ordered_items."<br><br>";

$price = 25.00;

$ordered_items = 12;

// after this executes $price = 21.25

get_volume_discount( $price, $ordered_items);

echo "Price: ".$price."  Qty Ordered: ".$ordered_items."<br><br>";

$price = 25.00;

$ordered_items = 51;

// after this executes $price = 20.00

get_volume_discount( $price, $ordered_items);

echo "Price: ".$price."  Qty Ordered: ".$ordered_items."<br><br>";

?>

Here are some bonus questions about our new function.

1. What happens if we pass a literal? For example, get_volume_discount( 25.00, $ordered_items);
2. What value is returned by our new function in each of the examples above?
3. What are some of the drawbacks of writing a function that accesses parameters by reference rather than by value?

Take some time to play with the function and get the answers for yourself. Functions are used extensively in PHP, so the more you are comfortable with them, the better PHP developer you will become.

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