PHP Variables

Variables are "containers" for storing information. In PHP, a variable starts with the $ sign, followed by the name of the variable.

1. Declaring Variables

Unlike many other languages, PHP has no command for declaring a variable. It is created the moment you first assign a value to it.

<?php
    $txt = "Hello world!";
    $x = 5;
    $y = 10.5;
?>

PHP is Loosely Typed: You don't have to tell PHP which data type the variable is. PHP automatically associates a data type to the variable, depending on its value.

2. Naming Rules

To write valid PHP, your variable names must follow these specific rules:

A variable name must start with the $ sign.
A variable name must start with a letter or the underscore character.
A variable name cannot start with a number.
A variable name can only contain alpha-numeric characters and underscores (A-z, 0-9, and _ ).
Variable names are case-sensitive ($age and $AGE are different).

3. Variable Scope

Scope refers to the part of the script where a variable can be referenced/used. PHP has three different variable scopes:

A. Global Scope

A variable declared outside a function has a GLOBAL SCOPE and can only be accessed outside a function.

<?php
    $x = 5; // global scope

    function myTest() {
        // using x inside this function will generate an error
        echo "<p>Variable x inside function is: $x</p>";
    } 
    myTest();

    echo "<p>Variable x outside function is: $x</p>";
?>

B. Local Scope

A variable declared within a function has a LOCAL SCOPE and can only be accessed within that function.

<?php
    function myTest() {
        $y = 5; // local scope
        echo "<p>Variable y inside function is: $y</p>";
    } 
    myTest();

    // using y outside the function will generate an error
    echo "<p>Variable y outside function is: $y</p>";
?>

The global Keyword

The global keyword is used to access a global variable from within a function. To do this, use the global keyword before the variables (inside the function):

<?php
    $x = 5;
    $y = 10;

    function myTest() {
        global $x, $y;
        $y = $x + $y;
    }

    myTest();
    echo $y; // outputs 15
?>

Static Keyword: Normally, all variables are deleted when a function is completed. However, sometimes we want a local variable NOT to be deleted. To do this, use the static keyword when you first declare the variable.

Key Points to Remember

Variables are created the moment they are assigned a value.
Always start with a $ followed by a letter or underscore.
Use meaningful names (e.g., $userEmail instead of $e).
Global variables are declared outside functions; **Local** variables are inside.
Use the global keyword to use global variables inside your functions.
PHP is loosely typed, so you don't declare types like `int` or `string`.

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