Introductory Tutorial
PHP is a tool that lets you create dynamic web pages. PHP-enabled
web pages are treated just like regular HTML pages and you can
create and edit them the same way you normally create regular HTML
pages.
What do I need?
In this tutorial we assume that your server has support for PHP
activated and that all files ending in .php are handled by
PHP. On most servers this is the default extension for PHP files,
but ask your server administrator to be sure. If your server
supports PHP then you don't need to do anything. Just create your
.php files and put them in your web directory and the server
will magically parse them for you. There is no need to compile
anything nor do you need to install any extra tools. Think of these
PHP-enabled files as simple HTML files with a whole new family of
magical tags that let you do all sorts of things.
Your first PHP-enabled page
Create a file named hello.php and in it put the following
lines:
<html><head><title>PHP Test</title></head>
<body>
<? echo "Hello World<P>"; ?>
</body></html>
|
The colours you see are just a visual aid to make it easier to see
the PHP tags and the different parts of a PHP expression. Note also
that this is not like a CGI script. The file does not need to be
executable or special in any way. Think of it as a normal HTML file
which happens to have a set of special tags available to you that
do a lot of interesting things.
This program is extremely simple and you really didn't need to
use PHP to create a page like this. All it does is display:
Hello World
If you tried this example and it didn't output anything, chances
are that the server you are on does not have PHP enabled. Ask your
administrator to enable it for you.
The point of the example is to show the special PHP tag format.
In this example we used <? to indicate the start of a PHP
tag. Then we put the PHP statement and left PHP mode by adding the
closing tag, ?>. You may jump in and out of PHP mode in
an HTML file like this all you want.
Something Useful
Let's do something a bit more useful now. We are going to check
what sort of browser the person viewing the page is using. In order
to do that we check the user agent string that the browser sends as
part of its request. This information is stored in a variable.
Variables always start with a dollar-sign in PHP. The variable we
are interested in is $HTTP_USER_AGENT. To display this
variable we can simply do:
<? echo $HTTP_USER_AGENT; ?> |
For the browser that you are using right now to view this page,
this displays:
Mozilla/4.61 [en] (X11; U; Linux 2.2.16.asl-7
i686)
There are many other variables that are automatically set by your
web server. You can get a complete list of them by creating a file
that looks like this:
Then load up this file in your browser and you will see a page full
of information about PHP along with a list of all the variables
available to you.
You can put multiple PHP statements inside a PHP tag and create
little blocks of code that do more than just a single echo. For
example, if we wanted to check for Internet Explorer we could do
something like this:
<?
if(strstr($HTTP_USER_AGENT,"MSIE")) {
echo "You are using Internet Explorer<br>";
}
?> |
Here we introduce a couple of new concepts. We have an "if"
statement. If you are familiar with the basic syntax used by the C
language this should look logical to you. If you don't know enough
C or some other language where the syntax used above is used, you
should probably pick up any introductory C book and read the first
couple of chapters. All the tricky string and memory manipulation
issues you have to deal with in C have been eliminated in PHP, but
the basic syntax remains.
The second concept we introduced was the strstr() function call.
strstr() is a function built into PHP which searches a string for
another string. In this case we are looking for "MSIE" inside
$HTTP_USER_AGENT. If the string is found the function returns true
and if it isn't, it returns false. If it returns true the following
statement is executed.
We can take this a step further and show how you can jump in and
out of PHP mode even in the middle of a PHP block:
<?
if(strstr($HTTP_USER_AGENT,"MSIE")) {
?>
<center><b>You are using Internet Explorer</b></center>
<?
} else {
?>
<center><b>You are not using Internet Explorer</b></center>
<?
}
?> |
Instead of using a PHP echo statement to output something, we
jumped out of PHP mode and just sent straight HTML. The important
and powerful point to note here is that the logical flow of the
script remain intact. Only one of the HTML blocks will end up
getting sent to the viewer. Running this script right now results
in:
You are not using Internet Explorer
Dealing with Forms
One of the most powerful features of PHP is the way it handles HTML
forms. The basic concept that is important to understand is that
any form element in a form will automatically result in a variable
with the same name as the element being created on the target page.
This probably sounds confusing, so here is a simple example. Assume
you have a page with a form like this on it:
<form action="action.php" method="POST">
Your name: <input type=text name=name>
You age: <input type=text name=age>
<input type=submit>
</form> |
There is nothing special about this form. It is a straight HTML
form with no special tags of any kind. When the user fills in this
form and hits the submit button, the action.php page is
called. In this file you would have something like this:
Hi <? echo $name?>. You are <? echo $age?> years old. |
It should be obvious what this does. There is nothing more to it.
The $name and $age variables are automatically set for you by PHP.