An Example Program

Let’s look at how a real-life PHP program integrates with an HTML form by creating the program convert.php, listed in Example 11-10. Type it in as shown and try it for yourself.

Example 11-10. A program to convert values between Fahrenheit and Celsius
<?php // convert.php
$f = $c = "";

if (isset($_POST['f'])) $f = sanitizeString($_POST['f']);
if (isset($_POST['c'])) $c = sanitizeString($_POST['c']);

if ($f != '')
{
    $c = intval((5 / 9) * ($f - 32));
    $out = "$f °f equals $c °c";
}
elseif($c != '')
{
    $f = intval((9 / 5) * $c + 32);
    $out = "$c °c equals $f °f";
}
else $out = "";

echo <<<_END
<html><head><title>Temperature Converter</title>
</head><body><pre>
Enter either Fahrenheit or Celsius and click on Convert

<b>$out</b>
<form method="post" action="convert.php">
Fahrenheit <input type="text" name="f" size="7" />
   Celsius <input type="text" name="c" size="7" />
           <input type="submit" value="Convert" />
</form></pre></body></html>
_END;

function sanitizeString($var)
{
    $var = stripslashes($var);
    $var = htmlentities($var);
    $var = strip_tags($var);
    return $var;
}
?>

When you call up convert.php in a browser, the result should look something like the screen grab in Figure 11-8.

The temperature conversion program in action
Figure 11-8. The temperature conversion program in action

To break the program down, the first line initializes the variables $c and $f in case they do not get posted to the program. The next two lines fetch the values of either the field named f or the one named c, for an input Fahrenheit or Celsius value. If the user inputs both, the Celsius is simply ignored and the Fahrenheit value is converted. As a security measure, the new function sanitizeString from Example 11-9 is also used.

So, having submitted either values or empty strings in both $f and $c, the next portion of code constitutes an if...elseif...else structure that first tests whether $f has a value. If not, it checks $c; if $c does not have a value either, the variable $out is set to the empty string (more on that in a moment).

If $f is found to have a value, the variable $c is assigned a simple mathematical expression that converts the value of $f from Fahrenheit to Celsius. The formula used is Celsius = (5 / 9) * (Fahrenheit – 32). The variable $out is then set to a message string explaining the conversion.

On the other hand, if $f is found not to have a value but $c does, a complementary operation is performed to convert the value of $c from Celsius to Fahrenheit and assign the result to $f. The formula used is Fahrenheit = (9 / 5) * Celsius + 32. As with the previous section, the string $out is then set to contain a message about the conversion.

In both conversions, the PHP intval function is called to convert the result of the conversion to an integer value. This isn’t necessary, but it looks better.

With all the arithmetic done, the program now outputs the HTML, which starts with the basic head and title and then provides some introductory text before displaying the value of $out. If no temperature conversion was made, $out will have a value of NULL and nothing will be displayed, which is exactly what we want when the form hasn’t yet been submitted. But if a conversion was made, $out contains the result, which is displayed.

After this, we come to the form, which is set to submit using the POST method to the file convert.php (the program itself). Within the form, there are two inputs for either a Fahrenheit or Celsius value to be entered. A submit button with the text “Convert” is then displayed, and the form is closed.

After outputting the HTML to close the document, we come finally to the function sanitizeString from Example 11-9.

Note

All the examples in this chapter have used the POST method to send form data. I recommend this, as it’s the neatest and most secure method. However, the forms can easily be changed to use the GET method, as long as values are fetched from the $_GET array instead of the $_POST array. Reasons to do this might include to make the result of a search bookmarkable or directly linkable from another page.