PHP Variables

Variables

In PHP, variables are denoted by a dollar sign ($) followed by the variable name. Variable names are case-sensitive and can contain letters, numbers, and underscores, but must start with a letter or an underscore.

$name = "John";
$age = 30;
$isStudent = true;

Constants

In PHP, constants are defined using the define() function or the const keyword. Constants are immutable and do not have a $ prefix.

define("PI", 3.14);
const GRAVITY = 9.81;

Magic Constants

PHP provides several predefined constants called magic constants that change depending on where they are used. Some common magic constants include:

echo "This is line number: " . __LINE__; // Outputs the current line number

echo "This file is located at: " . __FILE__; // Outputs the full path and filename of the file

echo "This directory is: " . __DIR__; // Outputs the directory of the file

Variable Scope

In PHP, variable scope refers to the context within which a variable is defined and can be accessed. The main types of variable scope in PHP are:

function localScope() {
    $localVar = "I am a local variable";
    echo $localVar; // Outputs: I am a local variable
}
localScope();
// echo $localVar; // This will cause an error because $localVar is not accessible outside the function

function staticScope() {
    static $count = 0;
    $count++;
    echo $count; // Outputs the number of times the function has been called
}
staticScope(); // Outputs: 1
staticScope(); // Outputs: 2
staticScope(); // Outputs: 3

Global Variables

Global variables are defined outside of any function and can be accessed throughout the script. However, to access a global variable within a function, you need to declare it as global using the global keyword or access it through the $GLOBALS array.

Using global keyword

$globalVar = "I am a global variable";
function testGlobal() {
    global $globalVar;
    echo $globalVar; // Outputs: I am a global variable
}
testGlobal();

Using $GLOBALS array

$globalVar = "I am a global variable";
function testGlobal() {
    echo $GLOBALS['globalVar']; // Outputs: I am a global variable
}
testGlobal();

Superglobals

PHP also provides several predefined superglobal variables that are available in all scopes throughout a script. Some common superglobals include:

// Example of using superglobals
echo $_SERVER['HTTP_USER_AGENT']; // Outputs the user agent string of the browser

// Example of using $_GET
// If the URL is http://example.com/?name=John
echo $_GET['name']; // Outputs: John

// Example of using $_POST
// If a form is submitted with a field named 'email'
echo $_POST['email']; // Outputs the submitted email address