Build Your Own Database Driven Website Using PHP & MySQL: Pt. 3

January 17, 2002

In Chapter 2, we learned how to use the MySQL database engine to store a list of jokes in a simple database (composed of a single table named "Jokes"). To do so, we used the MySQL command-line client to enter SQL commands (queries). In this chapter, we'll introduce the PHP server-side scripting language. In addition to the basic features we'll explore here, this language has full support for communication with MySQL databases.

As we've discussed previously, PHP is a server-side scripting language. This concept is not obvious, especially if you're used to designing pages with just HTML and JavaScript. A server-side scripting language is similar to JavaScript in many ways, as they both allow you to embed little programs (scripts) into the HTML of a Web page. When executed, such scripts allow you to control what will actually appear in the browser window with more flexibility than is possible using straight HTML.

The key difference between JavaScript and PHP is simple. JavaScript is interpreted by the Web browser once the Web page that contains the script has been downloaded. Meanwhile, server- side scripting languages like PHP are interpreted by the Web server before the page is even sent to the browser. And, once it's interpreted, the results of the script replace the PHP code in the Web page itself, so all the browser sees is a standard HTML file. The script is processed entirely by the server, hence the designation: server-side scripting language.

Let's look back at the today.php example presented in Chapter 1:

<html>
<head>
<title>Today's Date</title>
</head>
<body>
<p>Today's Date (according to this Web server) is
<?php
  echo( date("l, F dS Y.") );
?></p>
</body>
</html>

Most of this is plain HTML. The line between <?php and ?>, however, is written in PHP. <?php means "begin PHP code", and ?> means "end PHP code". The Web server is asked to interpret everything between these two delimiters, and to convert it to regular HTML code before it sends the Web page to the requesting browser. The browser is presented with something like this:

<html>
<head>
<title>Today's Date</title>
</head>
<body>
<p>Today's Date (according to this Web server) is
Wednesday, May 30th 2001.</p>
</body>
</html>

Notice that all signs of the PHP code have disappeared. In their place, the output of the script has appeared, and looks just like standard HTML. This example demonstrates several advantages of server-side scripting:

No browser compatibility issues. PHP scripts are interpreted by the Web server and nothing else, so you don't have to worry about whether the language you're using will be supported by your visitors' browsers.
Access to server-side resources. In the above example, we placed the date according to the Web server into the Web page. If we had inserted the date using JavaScript, we would only be able to display the date according to the computer on which the Web browser was running. Now, while this isn't an especially impressive example of the exploitation of server-side resources, we could just as easily have inserted some other information that would only be available to a script running on the Web server, for example, information stored in a MySQL database that runs on the Web server computer.
Reduced load on the client. JavaScript can significantly slow down the display of a Web page on slower computers, as the browser must run the script before it can display the Web page. With server-side scripting, this becomes the burden of the Web server machine.

Basic Syntax and Commands

PHP syntax will be very familiar to anyone with an understanding of C, C++, Java, JavaScript, Perl, or any other C- derived language. A PHP script consists of a series of commands, or statements, each of which is an instruction that the Web server must follow before it can proceed to the next. PHP statements, like those in the above-mentioned languages, are always terminated by a semicolon (;).

This is a typical PHP statement:

echo( "This is a <b>test</b>!" );

This statement invokes a built-in function called echo and passes it a string of text: This is a <b>test</b>! Built-in functions can be thought of as things that PHP knows how to do without us having to spell out the details. PHP has a lot of built-in functions that let us do everything from sending email, to working with information that's stored in various types of databases. The echo function, however, simply takes the text that it's given, and places it into the HTML code of the page at the current location. Consider the following (echo.php in the code archive):

<html>
<head>
<title> Simple PHP Example </title>
</head>
<body>
<p><?php echo("This is a <b>test</b>!"); ?></p>
</body>
</html>

If you paste this code into a file called echo.php (or echo.php3, if your Web host has not configured .php files to be recognized as PHP scripts) and place it on your Web server, a browser that views the page will see this:

<html>
<head>
<title> Simple PHP Example </title>
</head>
<body>
<p>This is a <b>test</b>!</p>
</body>
</html>

Notice that the string of text contained HTML tags (<b> and </b>), which is perfectly acceptable.

You may wonder why we need to surround the string of text with both parentheses and quotes. Quotes are used to mark the beginning and end of strings of text in PHP, so their presence is fully justified. The parentheses serve a dual purpose. First, they indicate that echo is a function that you want to call. Second, they mark the beginning and end of a list of "parameters" that you wish to provide, in order to tell the function what to do. In the case of the echo function, you only need to provide the string of text that you want to appear on the page. Later on, we'll look at functions that take more than one parameter (and we'll separate those parameters with commas), and we'll consider functions that take no parameters at all (for which we'll still need the parentheses, though we won't type anything between them).

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Build Your Own Database Driven Website Using PHP & MySQL: Pt. 3

Basic Syntax and Commands

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