Server Side Scripting Using PHP

PHP (PHP Hypertext Preprocessor) is one of the most popular server side scripting languages used for creating dynamic web pages. PHP usually runs on a web server and is available on different operating systems and platforms. It is an interpreted language — a PHP script is processed by the PHP engine each time it runs. PHP is an HTML embedded language, meaning PHP statements can be written within HTML code. When a PHP script runs, it processes the information and produces hypertext (HTML) as a result.

PHP was created by Rasmus Lerdorf, first in 1995, with the first official release in 1997. He is known as the Father of PHP, and continues to contribute to the project. PHP is an open-source project, completely free, and supported by a worldwide community of volunteers. It can access more than 20 different databases including MySQL, and can also output images, PDF files and Flash movies.

Client-side scripts execute faster since they run at the client, but a client-side language is dependent on the browser, and code like JavaScript is visible through "view source", making it less secure. PHP code, on the other hand, is executed on the server — only the output is sent to the client, so it is impossible for users to see the PHP source code.

• Easily embedded into HTML — simple to convert a static site into a dynamic one.
• Compatible with a wide variety of databases.
• An independent platform — runs across Windows, Linux or Mac servers.
• Supports most current web servers and interfaces easily with Apache/MySQL.
• Simple and easy to learn, with syntax similar to C++.
• Operates fast since it does not use a lot of system resources.
• Offers many levels of security to prevent malicious attacks.
• Free, open source, and legally usable for personal and commercial development.

A. Setting up the development environment — A PHP compatible web server and PHP interpreter must be installed, either separately or from a bundled package like WAMP (Windows, Apache, MySQL, PHP), XAMPP (X-OS, Apache, MySQL, PHP, PERL — works on any OS) or LAMP (Linux, Apache, MySQL, PHP). After installation, typing http://localhost in the browser confirms the server is running.

B. Writing and running the script — PHP scripts are plain text; only a text editor is needed. PHP scripting begins with <?php and ends with ?>. Files are saved with a .php extension in the web server's document root directory (e.g. C:\wamp\www for WAMP), and the output is viewed by typing the corresponding URL (e.g. http://localhost/example.php).

C. Combining HTML and PHP — PHP and HTML function together: PHP generates HTML, and HTML passes information to PHP. PHP code is embedded anywhere inside standard HTML using the tags <?php and ?>; anything outside these tags is read as plain HTML.

D. Comments in PHP — A comment is a line not read/executed as part of the program. PHP supports single line comments (//) and multiline comments (/* ... */).

E. Output statements — echo and print are the two basic constructs used to display output; both can output any data type, but only echo can take multiple comma-separated parameters. var_dump() displays both the data type and value of one or more variables.

A PHP variable name starts with the $ sign, followed by the name of the variable. Unlike other languages, a PHP variable does not need to be declared before use — it is created the first time a value is assigned to it, and PHP automatically determines its data type.

Naming rules:

• Must start with $, followed by a letter or underscore.
• Can contain only alpha-numeric characters and underscores (a–z, A–Z, 0–9, _).
• Variable names are case-sensitive ($name and $NAME are different).
• Punctuation such as commas, quotation marks, periods or spaces are not allowed.

PHP data types are grouped into Core Data Types — Integer, Float, Boolean, String — and Special Data Types — Null, Array, Object, Resource.

• Integer — whole numbers, positive or negative (e.g. -32, 32, 986); PHP does not support unsigned integers.
• Float/Double — fractional numbers (e.g. 123.56, 1.2e3).
• String — text within single or double quotes, e.g. "Apple", 'PHP'; the opening and closing quote type must match.
• Boolean — TRUE or FALSE, often used in conditional testing.
• Null — a variable is NULL if not set; can also be used to empty a variable.
• Array — holds multiple key/value pairs (indexed, associative or multidimensional).
• Object — can contain variables and functions; created from a class using new.
• Resource — a special variable holding references to external resources like a file handler or database object.

PHP operators are grouped as Assignment, Arithmetic, Relational, Logical, String and Combined operators.

Escape sequences — a single character headed by a backslash (\) used inside strings:

Conditional statements such as if, else, elseif and switch allow a program to execute different pieces of code, or none at all, depending on some condition.

The switch statement is similar to a series of if statements on the same expression, working exactly like switch in C++ — except PHP's switch can also accept strings as case identifiers.

The while loop executes the nested statements repetitively until the expression returns false. The do…while loop executes a block of code at least once, then repeats as long as the condition is true. The for loop is used when the number of iterations is known in advance.

continue skips the rest of the current loop iteration and jumps to the condition evaluation. break ends execution of the current for, foreach, while, do–while or switch structure; it accepts an optional numeric argument telling it how many nested structures to break out of.

An array in PHP is a collection of key and value pairs. Unlike C++, where only non-negative integers are used as keys, PHP allows either non-negative integers or strings as keys. PHP supports three types of arrays: Indexed, Associative and Multidimensional.

Arrays with a numeric index are called indexed arrays; by default the index starts from zero. The function array() creates an array, and print_r() prints an array in a single statement.

Arrays with named (string) keys are called associative arrays — similar to indexed arrays in functionality, but different in how the index is represented.

foreach is an extension to the for loop, used when the number of array elements is unknown. It iterates through each key/value pair until the end of the array, and has two forms:

Just like other languages, PHP has both user-defined and built-in functions. A user-defined function declaration starts with the word function; naming rules for functions are the same as for variables.

PHP comes with many useful built-in functions:

Three-tier architecture is a client-server architecture in which the functionalities like user interface, application programs and data storage are separated into layers (tiers), usually located on physically separate computers, so that one layer can be modified without affecting the others.

• Tier-1 (Browser/Client) — the front end that renders the content received from the web server, which may be plain HTML or HTML embedded with JavaScript.
• Tier-2 (Web Server) — the middle, dynamic-content processing level (e.g. Apache running PHP). It accepts requests from the browser, runs the requested script and sends the output back as HTML.
• Tier-3 (Database) — the back-end DBMS (e.g. MySQL) that interprets and executes SQL commands sent from the web server and returns the result.

In PHP, forms are used to collect information from the client and pass it to the server. Global variables are always accessible from anywhere in the code — regardless of scope, they can be accessed from any function, class or file.

PHP contains predefined global arrays called superglobals, always available regardless of scope:

$_REQUEST collects submitted form data regardless of method; $_POST and $_GET collect data submitted with method="post" and method="get" respectively, and $_GET can also read parameters appended directly to a URL.

GET should never be used for sending passwords or other sensitive information.

Static web pages never change unless a new page is loaded; dynamic pages change every time they are loaded, based on what users do. A dynamic website is built automatically by the web server, pulling information from the database into a web page template on each request.

1. Open a connection to MySQL using mysql_connect($server,$user_name,$password), which returns a link identifier or NULL on error.
2. Specify the database to open using mysql_select_db($database), which returns true or false.
3. Retrieve/insert data: execute the SQL query with mysql_query(), then populate rows into an array with mysql_fetch_array().
4. Close the connection using mysql_close($db_handle).

As too many open MySQL connections can cause problems, it is good practice to close the connection once all queries are executed.

Once connected, DDL and DML statements are prepared as strings and executed with mysql_query():