PHP Variables

Variable Basics

<?php
$age = 25;
$name = "Alice";
?>

<?php
$name = "John";
$Name = "Jane";
// Different variables!
?>

<?php
$user_name = "Valid";
$_temp = "Valid";
$user2 = "Valid";
?>

<?php
$var = "text";
$var = 123; // Now a number
?>

🔹 Creating Variables

How to declare and assign values to variables:

<?php
// Creating variables with different data types

// String (text)
$name = "Alice";
$city = 'New York';

// Integer (whole number)
$age = 25;
$quantity = 100;

// Float (decimal number)
$price = 19.99;
$temperature = 98.6;

// Boolean (true/false)
$isActive = true;
$hasDiscount = false;

// Display the variables
echo "Name: $name<br>";
echo "City: $city<br>";
echo "Age: $age<br>";
echo "Price: $$price<br>";
echo "Active: " . ($isActive ? "Yes" : "No");
?>
                            

🔹 Variable Naming Conventions

Best practices for naming variables:

<?php
$userName = "Valid";      // camelCase
$user_name = "Valid";     // snake_case (common in PHP)
$_private = "Valid";      // Starting with underscore
$user2 = "Valid";         // Numbers allowed (not at start)
$UserName = "Valid";      // PascalCase
?>

<?php
$2user = "Invalid";       // Can't start with number
$user-name = "Invalid";   // Hyphens not allowed
$user name = "Invalid";   // Spaces not allowed
$user@name = "Invalid";   // Special characters not allowed
?>

🔹 Variable Scope

Understanding where variables can be accessed:

🔸 Local Scope

<?php
function myFunction() {
    $localVar = "I'm local";
    echo $localVar;  // Works inside function
}

myFunction();
// echo $localVar;  // ERROR: Not accessible outside function
?>
                            

🔸 Global Scope

<?php
$globalVar = "I'm global";

function showGlobal() {
    global $globalVar;  // Must declare as global
    echo $globalVar;
}

showGlobal();  // Outputs: I'm global
echo $globalVar;  // Also works outside function
?>
                            

🔸 Static Variables

<?php
function counter() {
    static $count = 0;  // Retains value between calls
    $count++;
    echo "Count: $count<br>";
}

counter();  // Count: 1
counter();  // Count: 2
counter();  // Count: 3
?>
                            

🔹 Variable Operations

Working with variables in different ways:

<?php
// Assignment
$x = 10;
$y = 20;

// Arithmetic operations
$sum = $x + $y;
$difference = $y - $x;
$product = $x * $y;
$quotient = $y / $x;

echo "Sum: $sum<br>";
echo "Difference: $difference<br>";
echo "Product: $product<br>";
echo "Quotient: $quotient<br>";

// String concatenation
$firstName = "John";
$lastName = "Doe";
$fullName = $firstName . " " . $lastName;
echo "Full Name: $fullName<br>";

// Increment/Decrement
$counter = 5;
$counter++;  // Now 6
$counter--;  // Back to 5
echo "Counter: $counter";
?>
                            

🔹 Variable Variables

Using the value of one variable as the name of another:

<?php
// Variable variables (advanced concept)
$varName = "message";
$$varName = "Hello World!";

// Now $message exists with value "Hello World!"
echo $message;  // Outputs: Hello World!

// Practical example
$fruit = "apple";
$apple = "A delicious red fruit";

echo $$fruit;  // Outputs: A delicious red fruit
?>
                            

🔹 Checking Variables

Useful functions to check variable status:

<?php
$name = "Alice";
$age = 25;
$empty = "";

// isset() - Check if variable is set and not null
if (isset($name)) {
    echo "Name is set<br>";
}

// empty() - Check if variable is empty
if (empty($empty)) {
    echo "Variable is empty<br>";
}

// is_null() - Check if variable is null
$nullVar = null;
if (is_null($nullVar)) {
    echo "Variable is null<br>";
}

// var_dump() - Display variable information
var_dump($age);  // int(25)

// gettype() - Get variable type
echo "<br>Type: " . gettype($name);  // string
?>
                            

🔹 Practical Variable Example

A real-world example using variables:

<?php
// E-commerce product example

// Product information
$productName = "Wireless Headphones";
$regularPrice = 99.99;
$discountPercent = 20;
$quantity = 2;
$taxRate = 0.08;

// Calculate discount amount
$discountAmount = $regularPrice * ($discountPercent / 100);

// Calculate discounted price
$salePrice = $regularPrice - $discountAmount;

// Calculate subtotal
$subtotal = $salePrice * $quantity;

// Calculate tax
$taxAmount = $subtotal * $taxRate;

// Calculate final total
$total = $subtotal + $taxAmount;

// Display receipt
echo "<h3>Order Receipt</h3>";
echo "<p>Product: $productName</p>";
echo "<p>Regular Price: $$regularPrice</p>";
echo "<p>Discount: $discountPercent% (-$$discountAmount)</p>";
echo "<p>Sale Price: $$salePrice</p>";
echo "<p>Quantity: $quantity</p>";
echo "<p>Subtotal: $$subtotal</p>";
echo "<p>Tax (8%): $$taxAmount</p>";
echo "<p><strong>Total: $$total</strong></p>";
?>