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Lesson 2: Syntax in PHP, Declaring Constants and Variables in PHP

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By Tan Nguyen

December 07, 2024

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In any programming language, understanding basic syntax, variables, and constants is foundational. PHP is no exception. This lesson delves into these core concepts.

Basic PHP Syntax

PHP's syntax is akin to that of the C language, offering familiarity to those experienced with C.

  • Embedding PHP in Files: PHP code can be placed anywhere within a .php file.
  • PHP Tags: Code blocks start with <?php and end with ?>. While short tags <? are available, their use is discouraged.
  • Pure PHP Files: If a file contains only PHP code, it's advisable to omit the closing ?> tag to prevent unintended whitespace or new lines after the closing tag.
  • Statement Termination: Each PHP statement should end with a semicolon ;.

Example:

php
<!DOCTYPE html> <html> <body>    <h1>My first PHP page</h1>    <?php    echo "Hello World!"; // Outputs data to the web browser    ?> </body> </html>

Comments in PHP

Comments are non-executable parts of the code, serving to explain and document. They assist in code maintenance and clarity.

  • Single-line Comments:
    • // This is a single-line comment
    • # This is also a single-line comment
  • Multi-line Comments:
    • /* This is a multi-line comment spanning multiple lines */

Declaring Variables in PHP

Variables are used to store data and are prefixed with the $ symbol.

  • Naming Rules:
    • Must start with a letter or underscore (_).
    • Can contain letters, numbers, and underscores.
    • Case-sensitive ($Variable and $variable are distinct).

Example:

php
<?php $greeting = "Hello, World!"; echo $greeting; // Outputs: Hello, World! ?>

Variable Scope

The scope determines where a variable can be accessed.

  • Global Scope: Variables declared outside functions.
  • Local Scope: Variables declared within functions.
  • Superglobals: Built-in variables accessible everywhere (e.g., $_GET, $_POST).

Dynamic Variable Names

PHP allows dynamic variable naming using variable variables.

Example:

php
<?php $varName = 'dynamic'; $$varName = 'Variable'; echo $dynamic; // Outputs: Variable ?>

Declaring Constants in PHP

Constants are immutable values defined using the define() function or the const keyword (PHP 5.3+).

  • Naming Rules:
    • By convention, constants are uppercase.
    • Must start with a letter or underscore.
    • Can contain letters, numbers, and underscores.

Example:

php
<?php define("SITE_NAME", "TanHongIT"); echo SITE_NAME; // Outputs: TanHongIT ?>

Magic Constants

PHP provides predefined constants that change based on their context.

  • __LINE__: Current line number.
  • __FILE__: Full path and filename.
  • __DIR__: Directory of the file.
  • __FUNCTION__: Function name.
  • __CLASS__: Class name.
  • __METHOD__: Class method name.
  • __NAMESPACE__: Current namespace.

Understanding these fundamentals is crucial for effective PHP programming. They form the building blocks upon which more complex concepts are built.

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