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PHP Variables (Introduction)

Lesson TL;DRTopic 2: PHP Variables (Introduction) 📖 6 min read · 🎯 beginner · 🧭 Prerequisites: insertselectupdatedeletequeries, javascriptbasics Why this matters Here's the thing — every PHP program you write ...
6 min read·beginner·php · variables · data-types · variable-scope

Topic 2: PHP Variables (Introduction)

📖 6 min read · 🎯 beginner · 🧭 Prerequisites: insert-select-update-delete-queries, javascript-basics

Why this matters

Here's the thing — every PHP program you write will need to remember stuff. A user's name, their age, a price, a message to display on screen. Without a way to store that information temporarily, your code can't do much. In PHP, we use variables for this, and they all start with a $ sign — PHP's way of saying "this is a container holding a value." Once you understand how to create and use variables, you unlock the ability to write PHP that actually does something real.

What You'll Learn

  • How to declare and name PHP variables using the $ syntax
  • The five core PHP data types: integers, floats, strings, booleans, and arrays
  • How to use variables in arithmetic operations, string concatenation, and function calls
  • The difference between global scope, local scope, and superglobal scope
  • How to read form input using PHP's built-in superglobal variables $_GET, $_POST, and $_SERVER

The Analogy

Think of PHP variables as labeled storage bins in a warehouse. Each bin has a name written on the outside — that name is the variable name — and whatever you place inside is the value. You can swap the contents of a bin at any time, and any worker on the warehouse floor (any line of your script) can grab a bin by its label and use what's inside. Some bins live on the main floor and anyone can reach them (global variables); others are locked inside a foreman's office and only that foreman can touch them (local variables); and a handful of special bins are mounted on the ceiling, visible from every corner of the warehouse no matter what (superglobals).

Chapter 1: What Are Variables?

A variable is a named container that holds data which can be used and manipulated throughout a program. In PHP, variables are essential for storing numbers, strings, arrays, and more.

Declaring Variables

Variables in PHP are declared using the $ symbol followed by the variable name. Rules for naming:

  • Must start with a letter or an underscore (_)
  • Can contain letters, numbers, and underscores after the first character
  • Are case-sensitive — $name and $Name are two different variables
<?php

$variableName = "value";

?>

Example Variables

<?php

// Integer variable
$age = 25;

// String variable
$name = "Alice";

// Float variable
$price = 19.99;

// Boolean variable
$isMember = true;

?>

Chapter 2: Variable Types

PHP supports five core data types:

  1. Integers — Whole numbers (e.g., 1, 2, 3)
  2. Floats — Numbers with decimal points (e.g., 1.23, 4.56)
  3. Strings — Sequences of characters (e.g., "Hello, World!")
  4. Booleans — true or false values
  5. Arrays — Ordered collections of values

Example of All Five Data Types

<?php

// Integer
$age = 30;

// Float
$height = 5.9;

// String
$name = "John Doe";

// Boolean
$isStudent = false;

// Array
$colors = array("red", "green", "blue");

?>

PHP is a loosely typed language — you never have to declare the type explicitly. PHP infers it from the value you assign.

Chapter 3: Working with Variables

Variables become useful the moment you combine them: in calculations, in text assembly, and as arguments to functions.

Arithmetic Operations

<?php

$number1 = 10;
$number2 = 5;

$sum        = $number1 + $number2;  // Addition
$difference = $number1 - $number2;  // Subtraction
$product    = $number1 * $number2;  // Multiplication
$quotient   = $number1 / $number2;  // Division

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

?>

String Concatenation

PHP uses the dot operator (.) to join strings — not the + operator like JavaScript.

<?php

$firstName = "John";
$lastName  = "Doe";

$fullName = $firstName . " " . $lastName; // Concatenation using the dot operator

echo "Full Name: " . $fullName;

?>

Using Variables in Functions

<?php

function greet($name) {
    return "Hello, " . $name . "!";
}

$personName = "Alice";
echo greet($personName);

?>

Chapter 4: Variable Scope

Where you declare a variable determines where you can use it. PHP has three scope levels:

  1. Global Scope — Variables declared outside any function. Accessible throughout the script, but not automatically inside functions.
  2. Local Scope — Variables declared inside a function. Only accessible within that function; they disappear when the function returns.
  3. Superglobal Scope — Built-in PHP variables accessible from any scope, inside or outside functions (e.g., $_POST, $_GET).

Example of Variable Scope

<?php

$globalVar = "I am a global variable";

function testScope() {
    $localVar = "I am a local variable";
    echo $localVar; // Accessible here
}

testScope();

echo $globalVar; // Accessible here

// echo $localVar; // ERROR — $localVar is not accessible outside testScope()

?>

If you genuinely need a global variable inside a function, PHP provides the global keyword — but leaning on it too often is a design smell. Prefer passing values as function arguments instead.

Chapter 5: Superglobal Variables

PHP ships with a set of built-in superglobal arrays that are automatically available in every scope of every script. The three you'll use most often:

  • $_GET — Collects data sent via URL query parameters (e.g., ?name=Alice)
  • $_POST — Collects data submitted via an HTML form using the HTTP POST method
  • $_SERVER — Contains server and execution environment information: headers, file paths, the request method, and more

Example: Reading a Form Submission with Superglobals

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Superglobal Example</title>
</head>
<body>

    <form method="post" action="">
        <label for="name">Name:</label>
        <input type="text" id="name" name="name" required>
        <button type="submit">Submit</button>
    </form>

    <?php
    if ($_SERVER["REQUEST_METHOD"] == "POST") {
        $name = $_POST['name'];
        echo "Hello, " . $name . "!";
    }
    ?>

</body>
</html>

How it works step by step:

  1. The HTML form submits via POST to the same page (action="").
  2. PHP checks $_SERVER["REQUEST_METHOD"] to confirm a POST submission occurred.
  3. It reads the submitted name field from $_POST['name'].
  4. It echoes a greeting back to the user.

🧪 Try It Yourself

Task: Create a PHP script that acts as a simple tip calculator.

  1. Declare two variables: $billTotal (a float, e.g., 48.50) and $tipPercent (an integer, e.g., 18).
  2. Calculate the tip amount and the grand total.
  3. Echo both values to the browser with labels.

Success criterion: You should see two lines in the browser — something like:

Tip amount: $8.73
Grand total: $57.23

Starter snippet:

<?php

$billTotal  = 48.50;
$tipPercent = 18;

$tipAmount  = $billTotal * ($tipPercent / 100);
$grandTotal = $billTotal + $tipAmount;

echo "Tip amount: $" . number_format($tipAmount, 2) . "<br>";
echo "Grand total: $" . number_format($grandTotal, 2);

?>

Try changing $billTotal and $tipPercent to different values and reload to verify the math updates correctly.

🔍 Checkpoint Quiz

Q1. Which of the following is a valid PHP variable name?

A) $1stName B) $_firstName C) $first-name D) $first name

Q2. What will the following code print?

<?php
$x = 10;
$y = 3;
echo $x . $y;
?>

A) 13 B) 103 C) 30 D) A runtime error

Q3. Given this code, what happens when testScope() is called and then echo $localVar runs outside it?

<?php
function testScope() {
    $localVar = "only here";
}
testScope();
echo $localVar;
?>

A) It prints "only here" B) It prints nothing (empty string) C) PHP throws an undefined variable notice / warning D) It prints null

Q4. A user submits a login form via HTTP POST. Which superglobal and key would you use to read their submitted username field?

A) $_GET['username'] B) $_POST['username'] C) $_SERVER['username'] D) $_REQUEST_BODY['username']

A1. B — $_firstName is valid. Variable names must start with a letter or underscore; $1stName starts with a digit, $first-name contains a hyphen (not allowed), and $first name contains a space.

A2. B — The dot operator concatenates strings, so 10 and 3 are joined as strings, producing "103", not the arithmetic sum 13.

A3. C — $localVar is in local scope inside testScope(). Outside the function it is undefined, so PHP emits an undefined variable notice (and outputs nothing for that variable).

A4. B — Form data submitted via the method="post" attribute is available in $_POST. $_GET is for URL query parameters; $_SERVER holds server/environment metadata; there is no $_REQUEST_BODY superglobal in PHP.

🪞 Recap

  • PHP variables are declared with a $ prefix, are case-sensitive, and must start with a letter or underscore.
  • PHP's five core data types are integers, floats, strings, booleans, and arrays — the type is inferred automatically.
  • The dot operator (.) concatenates strings; arithmetic operators (+, -, *, /) work on numbers.
  • Variable scope is either global (outside functions), local (inside functions), or superglobal (everywhere).
  • $_GET, $_POST, and $_SERVER are superglobals used to read URL parameters, form submissions, and server information respectively.

📚 Further Reading

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