1 About this module

• Prerequisites:
• Objectives: This module explores how PHP scripting fits in the big picture of web related “stuff.”

2 Concepts and Terms

2.1 Client

A client, in general, refers to an entity that sends requests and usually expects an answer/response from the server designated by the request.

2.2 Server

A server, in general, refers to an entity that answers requests with a reply. A server does not actively do much until it receives a request from a client.

2.3 HTTP

“HTTP” stands for HyperText Transport Protocol. This is a networking protocol that allows a client to send a request and the server designated by the request to reply with a response.

An HTTP client is commonly known as a browser or web browser. Firefox, Chrome, Internet Explorer are commonly used browsers. When an end user types an URL on the address bar and presses the enter key, the web browser sends an HTTP request to the designated server.

Do not confuse HTTP with HTML. HTTP is a “transport protocol”, which means it is only specifying the standard that permits a client and a server interact. However, it does not, in any way, specifies how content is displayed.

2.4 HTML

“HTML” stands for HyperText Mark-up Language. This is a standard that specifies how content that is transmitted in plain text should be rendered by a browser.

Note that a document written in HTML does not need to be from a server. One can store HTML documents on a local hard disk drive and ask the browser to display (render) the documents.

You can view an HTML document as a collection of instructions to display content on the screen.

3 Static content

In CISW300, you learned how to create HTML documents. Combined with CSS (cascading stylesheets), HTML documents can specify some pretty fancy content to display. However, an HTML document that is stored on a server is static, it does not change. If you load an HTML document 200 times, it will load the same content all 200 times. This does not mean the document will look exactly the same all 200 times because Javascript, a client-side scripting language, can specify content that is dynamic.

Static content is okay in some applications. However, in general, content is dynamic. Most web sites need the same URL to display something different depending on many factors.

For example, an LMS (learning management system) URL should display an up-to-date score as assessments are being graded. The same URL to a grade book page should display updated content as an instructor grades entries in the grade book.

Likewise, an e-seller URL should also display an up-to-date price and stock status of an item. The same URL should load content that reflects the current price and stock status.

In the beginning of the Internet, most URLs specify static content. However, static content is fairly rare these days. Most URLs need to communicate with a database backend to display dynamic content that is based on up-to-date data stored in the database.

4 Where is the Database?

Most URLs that display dynamic content gets the raw data from a database. A database may not live on the same server as the HTTP server, but it is usually inaccessible to HTTP clients on the Internet for security reasons.

The database that provides dynamic data is accessible to an HTTP server.

5 PHP script in slow motion

• An URL specifying a PHP script on a server is triggered. This can be entered manually in the address bar, or a button associated with a PHP script is clicked, or a hyperlink is clicked.
• The URL is included in an HTTP request, and the request is sent by the HTTP client (web browser) to the server specified in the URL.
• Assuming the server exists and reachable, the HTTP server receives the HTTP request. Apache is a commonly used open sourced HTTP server.
• The HTTP server disects the HTTP request and figure out the file path to the PHP script, along with any parameters specified.
• The HTTP server follows the file path to retrieve the PHP script.
• Asssuming the PHP script is retrievable, the HTTP server runs the PHP interpreter to interpret and execute the code in the PHP file.
• As the PHP interpreter runs the code in the PHP file, an HTML document is created. As the PHP interpreter executes the script, resources on the server (machine) may be used, this includes a database server and many commands that are available on the server.
• Assuming the PHP interpreter successfully runs the PHP script, resulting in an HTML document, the HTML document is sent back to the HTTP client from the HTTP server as a response.
• When the HTTP client receives the response, the HTML content is extracted and used to render the content on the screen.

6 Can I run a PHP script manually?

Absolutely! If you have CLI (command line interface) access on a server machine and know the path to a PHP script, you can test run a PHP script manually.

On a CLI, type “php-cgi” preceeding the path of a PHP script to run it. For example, if “info.php” is in the current working directory, then the following command runs the script:

The output of the PHP interpreter is displayed on screen. It is an HTML document.

Running a PHP script manually is often helpful to debug a script that erroneously does not generate a proper HTML document.