PHP Language Core

  PHP Language Core

 Intended Audience

It is anticipated that the typical reader already has some web based

experience at least in terms of understanding the concepts of a web

server and creating HTML based content. While prior programming and scripting experience will be beneficial to the reader, this book is designed such that even the non-programmer can quickly get up to speed with PHP.

 The History of PHP

Every once in a while a person faces a particular problem or requirement to which there appears to be no existing solution. Faced with this problem the person decides to create a solution to provide the needed functionality.

Having developed the solution to their problem it then occurs to them that

others may need to solve the same problem, and they decide to make

their solution freely available to others who, in turn, can use and improve

on it. Within a short period of time many people adopt the technology

and work on it, adding new features they feel will be useful. The solution

soon grows beyond expectations in terms of features and is adopted by

more people than the original creator could ever have imagined.

 The Creation of PHP

The first version of what came to be known as PHP was created in 1995

by a man named Rasmus Lerdof. Rasmus, now an engineer at Yahoo!,

needed something to make it easier to create content on his web site,

something that would work well with HTML, yet give him power and

flexibility beyond what HTML could offer him. Essentially, what he needed was an easy way to write scripts that would run on his web server both to create content, and handle data being passed back to the server from the web browser. Using the Perl language, he created some technology that gave him what he needed and decided to call this technology “Personal Home Page/Forms Interpreter”. The technology provided a convenient way to process web forms and create content.

The name “Personal Home Page/Forms Interpreter” was later shortened

to PHP/FI and eventually renamed to represent “PHP: Hypertext

Preprocessor”. The name is said to be recursive because the full name also includes the acronym “PHP” - an odd geeky joke that is common in technology circles when people have trouble naming things. GNU is another recursive name that represents “GNU’s Not Unix”. 

PHP/FI version 1.0 was never really used outside of Rasmus’ own web site. With the introduction of PHP/FI 2.0 this began to change. When PHP 3 was released in 1997, adoption of PHP exploded beyond all belief.

PHP 3 Hits the Big Time

By the time 1997 arrived the number of web sites on the internet was

growing exponentially and most of these web sites were being

implemented using the Apache web server. It was around this time that

Andy Gutmans and Zeev Suraski launched the PHP 3 project, a project

designed to take PHP to the next level. One of the key achievements of

the PHP 3 project was to implement PHP as a robust Apache Module.

PHP 3 was implemented using a modular approach that made it easy for others to extend functionality, and also introduced the first elements of object-orientation that would continue to evolve through subsequent releases.

The combination of PHP 3 and Apache quickly lead to the widespread

adoption of PHP, and it is commonly estimated that, at its peak adoption level, PHP3 was used to power over 10% of all web sites on the internet.

 PHP 4 - Optimization, Scalability and More

With PHP 4 Andi Gutmans and Zeev Suraski once again re-architected

PHP from the ground up. PHP 4 was built upon a piece of technology

called the Zend Engine. The move to the Zend Engine brought about a

number of key improvements in PHP:

 Support for other web servers (Microsoft’s Internet Information

Server (IIS) being of particular significance).

 Improved memory handling to avoid memory leaks (one of the

most difficult types of problems to isolate in a program).

 Improved efficiency and performance to support large scale,

complex, mission critical enterprise application development

using PHP.

In addition PHP 4 also built on the earlier Object Oriented

Programming features of PHP 3 with the introduction of classes.

 PHP 5 - Object Orientation, Error Handling and XML

The main, though far from only, feature of PHP 5 is the improved support

for Object Oriented Programming (OOP). In addition, PHP 5 introduced

some features common in other langauges such as Java like try/catch

error and exception handling.

PHP 5 also introduced new extensions aimed at easing the storage and

manipulation of data. Significant new features include SimpleXML for

handling XML documents, and SQLite, an embedded basic and easy to

use database interface.

 What exactly is PHP?

PHP is an intuitive, server side scripting language. Like any other scripting

langauge it allows developers to build logic into the creation of web page

content and handle data returned from a web browser. PHP also contains

a number of extensions that make it easy to interact with databases,

extracting data to be displayed on a web page and storing information

entered by a web site visitor back into the database.

PHP consists of a scripting language and an interpreter. Like other

scripting languages, PHP enables web developers to define the behavior

and logic they need in a web page. These scripts are embedded into the

HTML documents that are served by the web server. The interpreter takes

the form of a module that integrates into the web server, converting the

scripts into commands the computer then executes to achieve the results

defined in the script by the web developer.

How Does PHP Work?

To develop an understanding of how PHP works it is helpful to first explore

what happens when a web page is served to a user’s browser.

When a user visits a web site or clicks on a link on a page the browser

sends a request to the web server hosting the site asking for a copy of the web page. The web server receives the request, finds the corresponding

web page file on the file system and sends it back, over the internet, to the user’s browser.

Typically the web server doesn’t pay any attention to the content

of the file it has just transmitted to the web browser. As far as the web

server is concerned the web browser understands the content of the web

page file and knows how to interpret and render it so that it appears as the web designer intended.

Now let’s consider what kind of web page content a web browser

understands. These days a web page is likely to consist of HTML, XHTML and JavaScript. 

The web browser contains code that tells it what to do with these types of content. For example, it understands the structure HTML in terms of rendering the page, and it has a JavaScript interpreter built in that knows how to execute the instructions in a JavaScript script.

A web browser, however, knows absolutely nothing about any PHP script that may be embedded in an HTML document. If a browser was served a web page containing PHP it would not know how to interpret that code.

Given that a web browser knows nothing about PHP in a web page, then clearly something has to be done with any PHP script in the page before it reaches the browser. This is where the PHP pre-processing module comes in. The PHP module is, as mentioned previously, integrated into the web server. 

The module tells the web server that when a page is to be served which contains PHP script (identified by special markers) that it is to pass that script to the PHP pre-processing module and wait for the PHP module to send it some content to replace that script fragment. 

The PHP processing module understands PHP, executes the PHP script

written by the web developer and, based on the script instructions, creates output that the browser will understand. The web server substitutes the content provided by the PHP pre-processor module in place of the PHP script in the web page and sends it to the browser where it is rendered for the user to view.

 Why is PHP so Useful?

In terms of web page content we have two extremes. At one extreme we have HTML which is completely static. There is very little that can be done with HTML to create dynamic content in a web page. At the other extreme we have scripting languages like JavaScript. JavaScript provides a powerful mechanism for creating interactive and dynamic web pages.

When talking about JavaScript it is important to understand that it is, by design, a client side scripting language. By this we mean that the script gets executed inside the user’s browser and not on the web server on which the web page originated. Whilst this is fine for many situations it is often the case that by the time a script reaches the browser it is then

either too late, or inefficient, to do what is needed. A prime example of this involves displaying a web page which contains some data from a database table. Since the database resides on a server (either the same physical server which runs the web server or on the same network as the web server connected by a high speed fiber network connection) it makes sense for any script that needs to extract data from the database to be executed on the server, rather than waiting until it reaches the browser. 

It is for this kind of task that PHP is perfectly suited. It is also fast and efficient (because the script is executed on the server it gets to take advantage of multi-processing, large scale memory and other such enterprise level hardware features.

In addition to the advantages of being a server side scripting language PHP is easy to learn and use. The fact that PHP works simlessly with HTML makes it accessible to a broad community of web designers. Perhaps one of the most significant advantages of PHP to some is the ease with which it interacts with the MySQL database to retrieve and store data.