Intended Audience

This article is intended for Web page developers and programmers considering PHP as an alternative Web page development tool. Both PHP's history and major features are discussed.

How PHP came into being

PHP started as a quick Perl hack written by Rasmus Lerdorf in late 1994. Over the next two to three years, it evolved into what we today know as PHP/FI 2.0. PHP/FI started to get a lot of users, but things didn't start flying until Zeev Suraski and Andi Gutmans suddenly came along with a new parser in the summer of 1997, leading to PHP 3.0. PHP 3.0 defined the syntax and semantics used in both versions 3 and 4.

Why yet another language?

People often ask " why invent yet another language; don't we have enough of them out there"? It is simply a matter of "the right tool for the right job". Many Web developers found that existing tools and languages were not ideal for the specific task of embedding code in markup. Those developers first collaborated with Rasmus and then later with Zeev and Andi, to develop a server-side scripting language which they felt would be ideal for developing dynamic Web-based sites and applications.

PHP was created with these particular needs in mind. Moreover, PHP code was developed for embedment within HTML. In doing so, it was hoped that benefits such as quicker response time, improved security, and transparency to the end user would be achieved. Considering that almost a million and a half sites are currently running PHP (at the time of this article's publication), it would appear that these developers were right.

PHP has evolved into a language, or maybe even an environment, that has a very specific range of tasks in mind. For this, PHP is, in Stig's humble opinion, pretty close to the ideal tool.

PHP 4 Architecture

PHP has undergone major architecture changes since version 3. First of all, the language parser itself has become a self-contained component called The Zend Engine. Secondly, PHP function modules, now called PHP extensions, are also basically self-contained. Thirdly, there is a Web server abstraction layer called SAPI that greatly simplifies the task of adding native support for new Web servers. SAPI also has the advantage of increasing PHP 4 stability, in addition to supporting multi-threaded Web servers.

The following figure presents PHP 4's architecture. The top border of the figure represents the programmer's interface (meaning all except for the Web server):

PHP 4 Architecture

SAPI currently has server implementations for Apache, Roxen, Java (servlet), ISAPI (Microsoft IIS and soon Zeus), AOLserver and of course CGI.

All of PHP's functions are part of one of the layers with a side facing up in the architecture figure. Most functions such as the MySQL support, are provided by an extension. Most extensions are optional. They can be linked into PHP at compile time or built as dynamically loadable extensions that can be loaded on demand.

Language Syntax

Most of PHP's syntax is borrowed from C, although there are elements borrowed from Perl, C++ and Java as well. This article assumes that you are familiar with C's syntax. However, don't panic if you're not.

Embedding PHP Code

To give you an idea of what embedding PHP would entail, consider the following three "hello world" examples, all of which will give the exact same output:

Example 1: HTML alone

Hello, World!

Example 2: PHP code alone

<?php print "Hello, World!"; ?>

Example 3: PHP embedded within HTML

<?php print "Hello,"; ?> World!

Web servers supporting PHP will, by default, scan a file in HTML mode. HTML code will be passed over to the browser as usual, up until the server happens upon a PHP line of code. In examples 2 and 3 above, the "<?php" tag informs the server that PHP code is to follow. The server then switches over to PHP mode in anticipation of a PHP command. The "?>" tag closes out the PHP mode with the server resuming its scanning in HTML mode once more.

Embedding code in this manner is, conceptually, a more fluid approach to designing a Web page because you are working within the output setting, namely an HTML page. Traditionally, you had to fragment the output (i.e. the header, body, footer etc..) and then put it into the code. Now we are inserting the code directly into the output.

From our lone example, however, one might come to ask, "So, what's the difference?" or "Why add extra code when HTML alone would do the trick?".

Read on!

Dynamic vs. Static Web pages

The "Hello, World" example we chose would certainly not require you to use PHP. That's because it is static, meaning its display will always remain the same. But what if you wanted to greet the world in any number of ways? Say, for example, "Bonjour, World!", or "Yo, World!" and so on.

Since HTML tags are purely descriptive they cannot function as a variable. Nor can they convey even the simplest of uncertainty such as a "Bonjour" or a "Yo". You need a command language to handle variability in a Web page. Based on either a conditional statement or direct user input, a command language can generate the "static" HTML necessary to correctly display a Web page's content.

Let us reconsider example #3. This time we want to let the user decide how to greet the world:

Example 4: PHP embedded within HTML revisited!

<?php print , ", "; ?> World!

From the above example, is assigned a value, and together with the comma and the word "World!", this value is sent to the browser.

Dynamic Web page design, however, is more than just about inserting variables. What if you wanted not only to greet the world in French, but also to present the page using the colors of the French flag?

Both a Web page's structure as well as its content can be customized. This means dynamic Web page programming can also entail on-demand Web page building.

No static, here!

Variables

In PHP, a variable does not require formal declaration. It will automatically be declared when a value is assigned to it. Variables are prefixed by a dollar sign: ().

Variables do not have declared types. A variable's type does not have to be fixed, meaning it can be changed over the variable's lifetime. The table below list's PHP's variable types:

In the following example, four variables are automatically declared by assigning a value to them:

<?php

 = 5;
 = "this is a string\n";
 = 'this is another "string"';
 = 37.2;

?>

Arrays

PHP arrays are a cross between numbered arrays and associative arrays. This means that you can use the same syntax and functions to deal with either type of array, including arrays that are:

• Indexed from 0
• Indexed by strings
• Indexed with non-continuous numbers
• Indexed by a mix of numbers and strings

In the example below, three literal arrays are declared as follows:

1. A numerically indexed array with indices running from 0 to 4.
2. An associative array with string indices.
3. A numerically indexed array, with indices running from 5 to 7.

<?php

 = array(2, 3, 5, 7, 11);
 = array("one" => 1, "two" => 2, "three" => 3);
 = array(5 => "five", "six", "seven");

printf("7: %d, 1: %d, 'six': %s\n", [3], ["one"], [6]);

?>

From the above example, the indices in the array1 are implicit, while the indices in array2 are explicit. When specifically setting the index to a number N with the => operator, the next value has the index N+1 by default. Explicit indices do not have to be listed in sequential order. You can also mix numerical and string indexes, but it is not recommended.

Conditionals and Looping Constructs

PHP includes if and elseif conditionals, as well as while and for loops, all with syntax similar to C. The example below introduces these four constructs:

<?php

// Conditionals

if () {
    print "a is true<BR>\n";
} elseif () {
    print "b is true<BR>\n";
} else {
    print "neither a or b is true<BR>\n";
}

// Loops

do {
     = test_something();
} while ();

while () {
    print "ok<BR>\n";
     = test_something();
}

for (220 = 0; 220 < 10; 220++) {
    print "i=220<BR>\n";
}

?>

Web Application Features

One of PHP's oldest features is the ability to make HTML form and cookie data available directly to the programmer. By default, any form entry creates a global PHP variable of the same name.

In the following example, a user name is retrieved and assigned to a variable. The name is then printed by the sub-routine "submit.php":

<FORM METHOD="GET" ACTION="submit.php">
What's your name? <INPUT NAME="myname" SIZE=3>
</FORM>

Submit.php

<?php
print "Hello, !";
?>

From the above example, note that variables can also be referenced from within double quoted strings.

For more information on strings, refer to our tutorial "Using Strings ".

Working With Cookies

You can set cookies in the browser from PHP using the setcookie() function. setcookie() adds headers to the HTTP response. Since headers must be inserted before the body, you will need to finish all of your setcookie() calls before any body output (usually HTML) is printed.

The following example uses a cookie to store a form value until your browser is terminated.


<?php

if (!) {
    print "What is your name? ";
    print "<FORM ACTION=\"/html2/modules.php\" METHOD=\"GET\">\n";
    print "<INPUT NAME=\"myname\" SIZE=20>\n";
    print "</FORM>";
    exit;
}

setcookie("myname", );

?>

For more information on using cookies, refer to our tutorial "Feedback Form with Cookies".

Built-in Variables

PHP has a number of built-in variables that give you access to your Web server's CGI environment, form/cookie data and PHP internals. Here are some of the most useful variables:

PHP internal variables

The Array and /html2/modules.php variables shown in the table below are specific to PHP:

HTTP Request Variables

Communication with MySQL

PHP and MySQL are often referred to as the "dynamic duo" of dynamic Web scripting. PHP and MySQL work very well together, in addition to the speed and features of each individual tool.

The following is a simple example of how to dump the contents of a MySQL table using PHP. The example assumes you have a MySQL user called "nobody" who can connect with an empty password from localhost:

MySQL Example

In the example below, PHP implements the following procedure:

• Connect to MySQL.
• Send a query.
• Print a table heading.
• Print table rows until end of the table has been reached.

<?php

//Connect to the database on the server's machine as
//user "Nobody".

 = mysql_connect("localhost", "nobody", "");
 = mysql_query("SELECT * FROM mytable", );
 = false;
print "<TABLE>\n";
do {
     = mysql_fetch_array();

    // Retrieve the next row of data.

    if (!is_array()) {
        break;
    }

// This part is done only on the first loop. It prints
// out the names of the fields as table headings. This
// ensures that the headings will only be printed if the 
// database returns at least one row.

    if (!) {
        print " <TR>";
        reset();
        while (list(Intro_php, ) = each()) {
            print "  <TH>Intro_php</TH>\n"
        }
        print " </TR>\n";
         = true;
    }
    print " <TR>\n";
    print "  <TD>";

// Instead of looping through the returned data fields,
// we use implode to create a string of all data items
// with the required HTML between them.

    print implode("</TD>\n  <TD>", );
    print " </TR>\n";
} while ();
print "</TABLE>\n";