PHP
is an excellent server-side technology for dynamic webpage generation.
Within a span of few years, it has gained immense popularity
among developers. This excellent 5-Part tutorial written by
Luigi Arlotta explains the basics of PHP programming.
His style is so simple that even absolute beginners would have
no trouble following this tutorial.
This tutorial is targeted at those users who may have never
programmed using any language before. Also programmers who have
experience in other languages can quickly browse through this
series and get their PHP code running within minutes...
Introduction
PHP
stands for "PHP: Hypertext Preprocessor". PHP is a
scripting language through which you can generate web pages
dynamically. PHP code is directly inserted in HTML documents
through specific TAGs declaring the code presence and then executed
when a client demands the page. PHP is a server-side language,
that's to say that PHP code is directly executed by the server,
while the client receives processed results as an HTML document.
This way of working is different from that of other scripting
languages as JavaScript, whose code is first loaded onto the
client machine and then executed by the client (the browser).
A few points to note about PHP programming -
| 1. |
All
compatibility problems existing between different browsers
are completely solved. The Client's browser, receives
a normal HTML page after the execution of a PHP code on
the server, and so it is always able to display it correctly
since it deals with only HTML. This does not happen with
scripting languages interpreted by the client's browser.
In this case the client downloads the script code and
tries to process it on the local machine. This procedure
works correctly only if the client is equipped with the
right software (generally called plugins or built-in support
in the browser).
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| 2. |
The
server side code processing sees to it that the script
code is never visible to the clients. That prevents "thefts"
of source code.
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| 3. |
The
server side code execution requires that your webserver
has been well configured. It must be able to recognize
HTML documents containing PHP code. In order to make this,
it is necessary to install a PHP engine and to edit some
lines in the webserver's configuration file.
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| 4. |
Server
side code processing needs resources (CPU time) for generating
the dynamic pages. A high number of client requests could
overload the server. But generally today's servers such
as Apache are made stable enough to handle a relatively
large number of clients.
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To
make the webserver differentiate between HTML documents containing
PHP code and normal HTML pages, .php, .php4 or
.phtml extensions are used in place of the .html
. These extensions can change according to the webserver configuration.
We shall stick to the standard .php extension.
Assume
that a client requests the following page (example1.php). The
source code for the file is shown below
<HTML>
<BODY>
<?
echo ("<H1>This is an example</H1>");
?>
</BODY>
</HTML> |
This page will be recognized, thanks to the extension from which
it is characterized and it will be processed as a HTML document
containing PHP code by the server. The code is interpreted before
the output is sent to the client. The webserver passes this
page through the PHP engine which processes it and executes
the PHP instructions in the code. It then substitutes the result
of the execution in place of the original PHP code. Once processed
by the PHP engine, the webserver then transmits this dynamically
generated page to the client. The client receives the following
HTML document
<HTML>
<BODY>
<H1>This is an example</H1>
</BODY>
</HTML> |
As you see, in the document sent by the web server to the client
there is no sign of PHP code. The code has been interpreted/processed
and replaced with HTML lines. The client will never be able
to deduce what code generated the particular HTML lines.
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PHP
instructions are placed inside special TAGs <?
and ?>
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These tags allow the PHP engine to distinguish the PHP syntax
from the rest of the document. It is possible to use different
TAGs editing the engine configuration file php.ini .
We shall stick to the most common ones - the ones shown above.
Necessary
Setup to Run PHP on your machine
To
start writing and testing your PHP scripts you need a webserver
and a PHP engine. The PHP engine must be combined with the webserver
so that the webserver can process the PHP code in your pages.
As far as webservers are concerned, on the Linux platform -
Apache rules. It is probably the most used webserver at the
moment. You can download Apache from www.apache.org.
The PHP engine (release 3 or 4) can here be downloaded from
www.php.net
You
could learn how to configure Apache to run PHP with the help
of the manual that you would be provided or you could search
for articles on Tips For Linux itself.
First steps
My
first PHP script consists of the classic program that displays
the famous " HELLO WORLD ". Copy the code shown below,
paste and save it in a file with the name helloworld.php
in your webserver standard directory (or in the webserver's
php files directory).
|
<HTML>
<BODY>
<?
echo ("<H1>Hello World!</H1>");
?>
</BODY>
</HTML>
|
Then
start your web browser and type the following in the address
bar -
http://127.0.0.1/helloworld.php
or
http://127.0.0.1/~username/helloworld.php
Note : In case you have not yet configured Apache Server
you could read Article No.
29 which explains Apache Server's configuration. Once you
Apache Server is configured, you could continue with this article.
If it works correctly you can proceed and read the rest of this
tutorial, otherwise you would have to do some more tweaking
and get PHP configured on your machine. If you can't solve the
problem, before abandoning, try to have a look to the software
documentation. Even try posting your problem on discussion forums
on the Web. You will definitely find a solution.
Variables
A variable is a block of memory, accessible through a name
chosen by the software developer, in which a value is stored.
This value, is usually given a default value at the beginning
of the application, and you can change that value during the
execution of the program. PHP requires variable names to
begin with the dollar ' $ ' character.
Variable names can be composed of uppercase and lowercase letters,
digits and underscores. But it is not possible to include spaces
or other special or reserved characters to define the names
of a variable. Remember that PHP is a case-sensitive language.
This means it distinguishes between uppercase and lowercase
variable names. For instance, if we write the following script.
The code below shows the case sensitive aspect of PHP.
<HTML>
<HEAD>
</HEAD>
<BODY>
<?
$VAR1 = 5;
echo ($var1);
?>
</BODY>
</HTML> |
We'll get an error message because the PHP interpreter does
not find $var1 variable (The variable defined initially was
$VAR1).
A
script involving variable declarations and displaying the values
of those variables is shown below. Read it carefully because
it contains some important points that we will discuss later.
<HTML>
<BODY>
<?
$website = "http://www.bitafterbit.com";
echo ("<BR>Surf to: $website");
echo ('<BR>Surf to: $website');
?>
</BODY>
</HTML> |
The program defines and prints the variable $website. Observe
that in the echo() function, whose purpose is to write
a string on the screen, two different kinds of inverted commas
are used - they are the single and double ones.
Important
: In this tutorial we will refer to the double inverted
commas (") as only inverted commas and we will refer to
single inverted commas (') as only quotes. The main difference
between these two types of syntax is that PHP interprets what
is enclosed in the inverted commas, while everything appearing
between quotes is considered a constant value and it is not
interpreted.
The example given above produces the following output
Surf
to: http://www.bitafterbit.com
Surf to: $website |
The text $website is in fact interpreted and replaced
with the value of the corresponding variable only in the first
echo() statement. This because in the first statement we have
used inverted commas to enclose the text to print. The result
of this first instruction is
| Surf
to: http://www.bitafterbit.com |
As
explained previously, in the second echo() statement,
where quotes have been used in place of inverted commas, the
enclosed text is not interpreted, because it is considered to
be a constant. The output of the second echo() instruction
therefore is
It
is important to take notice of this important characteristic
of PHP. It is different from other programming languages where
all that appears enclosed between inverted commas is considered
a constant value (a string). We will talk about strings in detail
in later articles when we'll discuss PHP's datatypes.
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©
Copyright by Luigi Arlotta. All rights reserved. Contact
the author for permissions.
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