Variables and Data Types

In programming, variables are symbolic names used to store data. Think of them as containers that hold different kinds of information. Data types classify the kind of values that a variable can hold, determining what operations can be performed on the data and how it is stored in memory.

Variables in PHP:

1. Declaration: In PHP, variables are declared by placing a dollar sign (`$`) before the variable name. For example, `$name`. Variable names are case-sensitive.
2. Assignment: You assign a value to a variable using the assignment operator (`=`). Example: `$age = 30;`
3. Dynamic Typing: PHP is a dynamically typed language, which means you don't need to explicitly declare the data type of a variable. PHP automatically determines the data type based on the value assigned to it. A variable can even change its data type during the script's execution.
4. Scope: Variables have a scope, which defines where they can be accessed (e.g., global, local, static).

PHP Data Types:

PHP supports several data types to categorize different kinds of data. These can be broadly divided into Scalar, Compound, and Special types.

A. Scalar Types (Single Value):

* `string`: Represents a sequence of characters. Can be enclosed in single quotes (`'...'`) or double quotes (`"..."`). Double quotes allow for variable parsing and escape sequences.
* Example: `"Hello World"`, `'PHP'`
* `integer`: Represents whole numbers (positive, negative, or zero) without a decimal point.
* Example: `10`, `-5`, `0`
* `float` (or `double`): Represents numbers with a decimal point or numbers in exponential form.
* Example: `3.14`, `-0.5`, `1.2e3` (which is 1200)
* `boolean`: Represents truth values. It can only be `TRUE` or `FALSE` (case-insensitive).
* Example: `true`, `false`

B. Compound Types (Multiple Values):

* `array`: Used to store multiple values in a single variable. Elements in an array can be of different data types. Arrays can be numerically indexed (starting from 0) or associatively indexed (using named keys).
* Example: `array(1, 2, 3)`, `["name" => "Alice", "age" => 30]`
* `object`: Instances of classes. Objects allow you to group related data and functions together.
* Example: `new MyClass()`

C. Special Types:

* `NULL`: Represents a variable that has no value. A variable is `NULL` if it has been assigned the constant `NULL`, has not been assigned a value yet, or has been unset.
* Example: `$myVar = NULL;`
* `resource`: Special variables that hold references to external resources (like database connections, file handles, image canvases). They are created by specific functions and are automatically freed by PHP when no longer needed.
* Example: `$fileHandle = fopen("file.txt", "r");`

Type Juggling and Type Casting:

PHP's dynamic typing often involves type juggling, where PHP automatically converts a value from one type to another as needed by the context (e.g., during arithmetic operations or comparisons). For example, if you add an integer to a string that looks like a number, PHP will convert the string to an integer.

Type casting is when you explicitly convert a value to a specific data type. This is done by placing the desired type in parentheses before the variable or value. Common casts include `(int)`, `(float)`, `(string)`, `(bool)`, `(array)`, `(object)`.

Understanding variables and data types is fundamental to writing effective PHP code, as it dictates how data is stored, manipulated, and processed within your applications.

<?php

// 1. Scalar Types

// String variable
$name = "John Doe";
$greeting = 'Hello, ' . $name . '!'; // Concatenation using '.'
echo "Name: " . $name . " (Type: " . gettype($name) . ")\n";
echo "Greeting: " . $greeting . " (Type: " . gettype($greeting) . ")\n\n";

// Integer variable
$age = 30;
$year = 2023;
echo "Age: " . $age . " (Type: " . gettype($age) . ")\n";
echo "Year: " . $year . " (Type: " . gettype($year) . ")\n\n";

// Float (Double) variable
$price = 19.99;
$pi = 3.14159;
echo "Price: " . $price . " (Type: " . gettype($price) . ")\n";
echo "Pi: " . $pi . " (Type: " . gettype($pi) . ")\n\n";

// Boolean variable
$isAdmin = true; // or TRUE
$isGuest = false; // or FALSE
echo "Is Admin: " . ($isAdmin ? 'true' : 'false') . " (Type: " . gettype($isAdmin) . ")\n";
echo "Is Guest: " . ($isGuest ? 'true' : 'false') . " (Type: " . gettype($isGuest) . ")\n\n";

// 2. Compound Types

// Array variable (indexed array)
$colors = ["red", "green", "blue"];
echo "First color: " . $colors[0] . " (Type: " . gettype($colors) . ")\n";
// Using var_dump for detailed array inspection
echo "Colors array details:\n";
var_dump($colors);
echo "\n";

// Array variable (associative array)
$person = [
    "firstName" => "Jane",
    "lastName" => "Smith",
    "age" => 25
];
echo "Person's first name: " . $person["firstName"] . " (Type: " . gettype($person) . ")\n";
echo "Person array details:\n";
var_dump($person);
echo "\n";

// Object variable (simple example)
class Car {
    public $brand;
    public function __construct($brand) {
        $this->brand = $brand;
    }
}
$myCar = new Car("Toyota");
echo "My car brand: " . $myCar->brand . " (Type: " . gettype($myCar) . ")\n";
echo "Car object details:\n";
var_dump($myCar);
echo "\n";

// 3. Special Types

// NULL variable
$unsetVar; // Declared but not assigned, implicitly NULL
$nullVar = NULL;

echo "Unset variable status: " . (is_null($unsetVar) ? 'NULL' : 'NOT NULL') . " (Type: " . gettype($unsetVar) . ")\n";
echo "Null variable status: " . (is_null($nullVar) ? 'NULL' : 'NOT NULL') . " (Type: " . gettype($nullVar) . ")\n";

// Example of changing type dynamically
$dynamicVar = "hello"; // string
echo "Dynamic var initial: " . $dynamicVar . " (Type: " . gettype($dynamicVar) . ")\n";
$dynamicVar = 123; // integer
echo "Dynamic var changed: " . $dynamicVar . " (Type: " . gettype($dynamicVar) . ")\n\n";

// Type Juggling and Type Casting

$numString = "10";
$numInt = 5;

// Type Juggling: PHP converts '$numString' to an integer for addition
$resultJuggling = $numString + $numInt;
echo "Juggling Result (\"10\" + 5): " . $resultJuggling . " (Type: " . gettype($resultJuggling) . ")\n";

// Explicit Type Casting: Convert string to integer
$resultCasting = (int)$numString + $numInt;
echo "Casting Result ((int)\"10\" + 5): " . $resultCasting . " (Type: " . gettype($resultCasting) . ")\n";

$boolToString = (string)true;
echo "Boolean true cast to string: '" . $boolToString . "' (Type: " . gettype($boolToString) . ")\n";

?>