\n\;

echo \

See what happens? The include files are tossed into the main file and then the whole thing is executed by PHP. Notice how the variable $title was defined before header.inc is referenced. Its value is made available to the code in header.inc; hence, the title of the page is changed. You can now use header.inc across all your PHP pages, and all you\ll have to do is change the value of $title from page to page.

Using a combination of includes, HTML, conditional statements, and loops, you can create complex variations from page to page with an absolute minimum of code. Includes become especially useful when used with functions, as we\ll see down the road.

On to the exciting world of data validation.

Page 2 — Simple Validation

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Imagine for a moment that we\ve got our database nicely laid out and we\re now requesting information from users that will be inserted into the database. Further, let\s imagine that you have a field in your database waiting for some numeric input, such as a price. Finally, imagine your application falling over in a screaming heap because some smart aleck put text in that field. MySQL doesn\t want to see text in that portion of your SQL statement - and it complains bitterly.

What to do? Time to validate.

Validation simply means that we\ll examine a piece of data, usually from an HTML form, and check to make sure that it fits a certain model. This can range from ensuring that a element is not blank to validating that an element meets certain criteria (for example, that a numeric value is stipulated or that an email address contains an @ for an email address).

Validation can be done on the server side or on the client side. PHP is used for server-side validation, while JavaScript or another client-based scripting language can provide client-side validation. This article is about PHP, so we\re going to concentrate on the server end of things. But if you\re looking for some ready-made, client-side validation scripts, check out the Webmonkey code library.

Let\s ignore our database for the moment and concentrate on PHP validation. If you wish, you can add additional fields to our employee database quite simply by using the MySQL ALTER statement - that is, if you want to commit to the values that we\ll validate.

There are several useful PHP functions we can use to validate our data, and they range from simple to highly complex. A simple function we could use might be strlen(), which tells us the length of the variable.

A more complex function would be ereg(), which uses full regular expression handling for complex queries. I won\t delve into the complexities of regex here, as entire books have been written on the subject, but I will provide some examples on the next page.

Let\s start with a simple example. We\ll check to see whether a variable does or does not exist.

     


     \>

     FIELD 1: \>

     


     FIELD 2: \>

     


     

     

     

The keys to this script are the nested conditional statements. The first checks to see whether the Submit button has been pressed. If it has, it goes on to check that both the variables $first and $last exist. The || symbol means \or\ and the ! symbol means \not.\ We could also rewrite the statement to say, \If $first does not exist or $last does not exist, then set $error to the following.\

Next, let\s extend things a little by checking to see whether a string is a certain length. This would be ideal for passwords, since you don\t want some lazy user entering a password of only one or two letters. You\d rather it be, say, six or more characters.

The function for this is, as you already know, strlen(). It simply returns a number equal to the number of characters in the variable being tested. Here, I modified the script above to check the length of $first and $last.

     


     
\>

     FIELD 1: \>

     


     FIELD 2: \>
    
     


     

     

     

Run this script and try entering six or fewer letters to see what happens. It\s simple yet quite effective.

Page 3 — Not-So-Simple Validation

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Let\s talk a bit about using regular expressions with the ereg() and eregi() functions. As I said earlier, these can be either quite complex or very simple, depending on what you need.

Using regular expressions, you can examine a string and intelligently search for patterns and variations to see whether they match the criteria you set. The most common of these involves checking whether an email address is valid (although, of course, there\s no fail-safe way of doing this).

Rather than delve into the mysteries of regular expressions, I\ll provide some examples. You can use the same form we created on the previous page - just paste in the lines below to see how they work.

First, let\s make sure that text only has been entered into a form element. This regular expression tests true if the user has entered one or more lowercase characters, from a to z. No numbers are allowed:

if (!ereg(\[a-Z]\, $first) || !ereg(\[a-Z]\, $last)) {

Now, let\s extend this expression to check whether the string is four to six characters in length. Using [[:alpha:]] is an easy way to check for valid alphabetic characters. The numbers in the braces check for the number of occurrences. And note that the ^ and $ indicate the beginning and end of the string.

if (!ereg(\^[[:alpha:]]{4,6}$\, $first) || !ereg(\^[[:alpha:]]{4,6}$\, $last)) {

Finally, let\s build a regular expression that will check an email address\ validity. There\s been plenty of discussion about the effectiveness of checking for email addresses in this way. Nothing\s completely foolproof, but what I have below works pretty well.

I took this gem from the PHP mailing list. It\s a great resource - use it. And yes, this is as scary as it looks.

	if (!ereg(\^[-!#$%&\*+\./0-9=?A-Z^_`a-z{|}~]+\.

\@\.

\[-!#$%&\*+\/0-9=?A-Z^_`a-z{|}~]+\.\.

\[-!#$%&\*+\./0-9=?A-Z^_`a-z{|}~]+$\, $last)) {

Don\t spend too much time looking at this. Just move on to the next page.

Page 4 — Functions

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Enjoy that last regex expression? Fun, wasn\t it? Wouldn\t it be even more fun to enter that chunk on a dozen different pages that need to process email addresses?! Think about the joy of finding a typo in that mess - and doing it a dozen times no less. But of course, there\s a better way.

Remember when we talked about include files earlier in this lesson? They\ll allow us to create a piece of code like the email checker and include it multiple times across several pages. This way, when we want to change the code, we need edit only one file, not many.

But if we want to get this done, we\ll have to use functions.

We\ve already used functions plenty of times. Every time we query the database or check the length of a string we\re using functions. These functions are built into PHP. If you\re a keen coder, you can extend PHP with your own customized functions. But that\s a bit advanced for this tutorial. Instead we\ll create functions that will reside within our PHP script.

A function is simply a block of code that we pass one or more values to. The function then processes the information and returns a value. The function can be as simple or complex as we like, but as long as we can pass a value in and get one out, we don\t really care how complex it is. That\s the beauty of functions.

Functions in PHP behave similarly to functions in C. When we define the functions, we must specify what values the function can expect to receive. It\s tricky to get a handle on at first, but it prevents weird things from happening down the road. This is done because the variables inside a function are known as private variables. That is, they exist only inside the function. You may, for instance, have a variable in your script called $myname. If you created a function and expected to use the same $myname variable (with the same value), it wouldn\t work. Alternatively, you could have the variable $myname in your script and also create another variable called $myname in your function, and the two would co-exist quite happily with separate values. I do not recommend doing this, however! When you come back and edit it six months later, you\ll be breaking things left and right. There are exceptions to this rule as with all things, but that\s outside the scope of this article.

So let\s create a function. We\ll start simply. We need to give the function a name and tell it what variables to expect. We also need to define the function before we call it.

That\s it! First, we created our function. Notice how we defined two new variables, called $first and $second. When we call the function, each variable is assigned a value based on the order in which it appears in the list - 4 goes to $first, 5 to $second. Then we simply added the two numbers together and returned the result. \Return\ here simply means to send the result back. At the end of the script we print the number 9.

Let\s create something that\s more useful to our database application. How about something that gracefully handles errors? Try this:

\;

	echo \The reported error was $error.\n
\;

	echo \Best you get hold of the site admin and let her know.\;

	die;

}



if (!$db = @mysql_connect(\localhost\,\user\, \password\)) {

	$db_error = \Could not connect to MySQL Server\;

	do_error($db_error);

}

?>

Before running this, try shutting down MySQL or using a bogus username or password. You\ll get a nice, useful error message. Observant readers will notice the @ symbol in front of mysql_connect(). This suppresses error messages so that you get the information only from the function. You\ll also see we were able to pass a variable into the function, which was defined elsewhere.

Remember that I said functions use their own private variables? That was a little white lie. In fact, you can make variables outside of a function accessible to the function. You might create a function to query a database and display a set of results over several pages. You don\t want to have to pass the database connection identifier into the function every time. So in this situation, you can make connection code available as a global variable. For example:

This is a basic function, but the point is that you don\t need to send $db through when you call the function - you can make it available using the word global. You can define other variables as global in this statement, just separate the variable names by a comma.

Finally, you can look like a real pro by using optional function variables. Here, the key is to define the variable to some default in the function, then when you call the function without specifying a value for the variable, the default will be adopted. But if you do specify a value, it will take precedence.

Confused? For example, when you connect to a database, you nearly always connect to the same server and you\ll likely use the same username and password. But sometimes you\ll need to connect to a different database. Let\s take a look.

Isn\t that cool? The variables used inside the function were defined when the function was defined. The first time the function is called, the defaults are used. The second time, we connect to a new host, but with the same username and password. Great stuff!

Think about where you could use other functions in your code. You could use them for data checking, performing routine tasks, and so on. I use them a lot when processing text for display on a Web page. I can check, parse, and modify the text to add new lines and escape HTML characters in one fell swoop.

Now all that\s left to do is to impart some words of wisdom.