Understanding PHP

PHP code is embedded within the HTML by bracketing it with   .

The code consists of one line, a call to the PHP function  date , which returns a

date in the format we've specified as a string, which is then output as part of the

HTML using the  echo  function. This ability to embed PHP inside HTML means

that you can continue to write your Web pages as you do now, and then embed

PHP into them to provide the dynamic elements.

This is perhaps a good point at which to highlight what is meant by a server side

language. The PHP code is executed on the server, not the browser. The source

of the page the browser receives back from the server is shown in Figure 2. You

can see that the PHP has been processed by the server, so no special software is

needed on the client to view pages with PHP elements   the intelligence lies at the

server end. This means you can use PHP with any browser   even simple, text only

ones like Lynx and w3m, or browsers in PDAs or set top boxes. So long as the

browser understands HTML it can display pages written in PHP.

PHP Syntax

In view of where it started, it's not surprising that PHP shares syntactical ideas

with Perl and C, and if you've programmed in either of these or you've done any

Unix shell programming, then PHP should be easy to pick up. PHP uses loosely

typed variables which don't need to be declared before first use, and evaluates

them according to context, so you can add a numeric string to a numeric to give

a numeric, and then manipulate the result as a string. The PHP online manual

gives a nice example of how a variable's type can change due to context. This is

shown in Figure 3.

One of the joys of PHP is that some variables are already set for you in any PHP

script. For example, the variable $HTTP_USER_AGENT will contain a string

identifying the browser which requested the page. So, for example, if you're using

Netscape 4.7 under NT4 to request the page, $HTTP_USER_AGENT will contain

the string  Mozilla/4.7 [en] (WinNT; I) . So if you need to generate HTML which

exploits browser specific features it's easy to do. Similarly, any cookies you have

set will automatically appear as variables in your script without you having to

write any code to extract them. As well as simple variables PHP also lets you

create arrays which act both like hash tables (associative arrays) and indexed

arrays (vectors), and these arrays can be multi dimensional.

PHP has a full range of comparative, logical and bitwise operators, auto increment

and decrement, and assignment operators. It has all the control structures you

might expect from a procedural language, including if/else, switch/case, for,

while (test before) and do (test after). You can write your own functions or use

the extensive range of functions provided as standard. Arguments can be parsed

by value or reference, and functions can return a value   or multiple values if you

use an array as the return variable. Variables are local to a function unless declared

global.

PHP also supports include files, both static and dynamic, allowing common code

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such as custom function libraries and HTML fragments to be embedded in your