PHP Variables and Data Types Explained — From Zero to Confident

<?php

// Declaring a string variable — text always goes inside quotes
$customerName = "Maria Santos";

// Declaring an integer variable — whole numbers, no quotes needed
$customerAge = 28;

// Declaring a float variable — numbers with decimal points
$accountBalance = 1042.75;

// Declaring a boolean variable — only two possible values: true or false
$isSubscribed = true;

// Outputting variables using echo
// The dot (.) is PHP's string concatenation operator — it joins strings together
echo "Customer: " . $customerName . "\n";   // Prints the customer's name
echo "Age: " . $customerAge . "\n";          // Prints the age as text
echo "Balance: $" . $accountBalance . "\n";  // Prints the balance
echo "Subscribed: ";                          // Starts the subscribed line
echo $isSubscribed ? "Yes" : "No";           // Ternary: prints Yes if true, No if false
echo "\n";

// PHP also lets you embed variables directly inside double-quoted strings
// This is called variable interpolation — PHP replaces the variable with its value
echo "Hello, $customerName! Your balance is $$accountBalance.\n";

// Checking what type PHP assigned using gettype()
echo "Type of customerAge: " . gettype($customerAge) . "\n";     // integer
echo "Type of accountBalance: " . gettype($accountBalance) . "\n"; // double (PHP calls floats 'double')
echo "Type of customerName: " . gettype($customerName) . "\n";   // string

?>

<?php

// ── SCALAR TYPES ──────────────────────────────────────────────

// STRING — any text wrapped in quotes
$productName = "Wireless Noise-Cancelling Headphones";

// INTEGER — whole numbers, positive or negative, no decimal point
$stockCount = 142;
$temperatureInCelsius = -5; // Integers can be negative

// FLOAT (also called double) — numbers with a decimal point
$productPrice = 89.99;
$taxRate = 0.08; // 8% tax stored as a decimal

// BOOLEAN — exactly two possible values: true or false (lowercase in PHP)
$isInStock = true;
$isDiscounted = false;

// ── COMPOUND TYPES ────────────────────────────────────────────

// INDEXED ARRAY — like a numbered shopping list, positions start at 0
$shoppingCart = ["Headphones", "Phone Case", "USB Cable"];

// ASSOCIATIVE ARRAY — like a form with labelled fields
$customerProfile = [
    "firstName"  => "James",      // key => value
    "lastName"   => "Okonkwo",
    "email"      => "james@example.com",
    "age"        => 34
];

// ── SPECIAL TYPES ─────────────────────────────────────────────

// NULL — the absence of a value; this variable exists but holds nothing
$discountCode = null;

// ── USING THE DATA ────────────────────────────────────────────

// Calculate final price with tax
$finalPrice = $productPrice + ($productPrice * $taxRate); // float arithmetic

echo "Product: $productName\n";
echo "Price: $" . number_format($finalPrice, 2) . "\n"; // number_format rounds to 2 decimal places
echo "In stock: " . ($isInStock ? "Yes" : "No") . "\n";
echo "Stock count: $stockCount units\n\n";

// Accessing an indexed array by position number
echo "First cart item: " . $shoppingCart[0] . "\n"; // Index 0 = first item
echo "Second cart item: " . $shoppingCart[1] . "\n";

// Accessing an associative array by key name
echo "Customer: " . $customerProfile["firstName"] . " " . $customerProfile["lastName"] . "\n";
echo "Email: " . $customerProfile["email"] . "\n";

// Checking for NULL with is_null()
if (is_null($discountCode)) {
    echo "No discount code applied.\n";
}

// var_dump shows type + value — incredibly useful for debugging
var_dump($isDiscounted); // Shows: bool(false)
var_dump($taxRate);      // Shows: float(0.08)

?>

<?php

// ── TYPE JUGGLING (PHP does this automatically) ────────────────

$quantityAsString = "5";     // This is a STRING — it has quotes
$bonusQuantity = 3;          // This is an INTEGER

// PHP sees you're adding and converts $quantityAsString to int automatically
$totalQuantity = $quantityAsString + $bonusQuantity;
echo "Total quantity: $totalQuantity\n";        // Outputs: 8
echo "Type: " . gettype($totalQuantity) . "\n"; // Outputs: integer (PHP juggled the type)

// A string that DOESN'T start with a number becomes 0 in arithmetic
$nonsenseString = "apples";
$result = $nonsenseString + 10;
echo "Nonsense arithmetic result: $result\n"; // Outputs: 10 ("apples" became 0)

// ── TYPE CASTING (YOU control the conversion) ─────────────────

$rawInput = "42.7 degrees"; // Imagine this came from a form submission

$asInteger = (int) $rawInput;   // Casts to int — stops at the decimal point
$asFloat   = (float) $rawInput; // Casts to float — grabs 42.7, ignores " degrees"

echo "\nRaw input: $rawInput\n";
echo "Cast to int: $asInteger\n";   // Outputs: 42
echo "Cast to float: $asFloat\n";   // Outputs: 42.7

// Casting to boolean — useful to know what PHP considers 'falsy'
$emptyString   = (bool) "";    // false — empty string is falsy
$zeroValue     = (bool) 0;     // false — zero is falsy
$zeroString    = (bool) "0";   // false — the string "0" is ALSO falsy (a classic gotcha!)
$nonEmptyStr   = (bool) "hello"; // true  — any non-empty, non-"0" string is truthy

echo "\n--- Boolean casts ---\n";
var_dump($emptyString);  // bool(false)
var_dump($zeroValue);    // bool(false)
var_dump($zeroString);   // bool(false) ← catches people out!
var_dump($nonEmptyStr);  // bool(true)

// ── LOOSE vs STRICT COMPARISON ────────────────────────────────

$formInput = "10"; // String from a form field
$dbValue   = 10;   // Integer from a database

// == (loose): converts types before comparing — both become 10, so true
$looseResult = ($formInput == $dbValue);
echo "\nLoose comparison (==): " . ($looseResult ? "true" : "false") . "\n"; // true

// === (strict): checks type AND value — string vs integer, so false
$strictResult = ($formInput === $dbValue);
echo "Strict comparison (===): " . ($strictResult ? "true" : "false") . "\n"; // false

?>