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PHP Variables, Constants, and Data Types Explained for Beginners

June 9, 2026 · Updated Sep 8, 2026 · 7 min read

PHP Variables, Constants, and Data Types Explained

In the previous article, we learned what PHP is and how it works. In this tutorial, we’ll explore three fundamental concepts every PHP developer must understand:

  • Variables
  • Constants
  • Data Types

These concepts form the foundation of PHP programming and are used in every PHP application, including WordPress and Laravel projects.

What is a Variable in PHP?

A variable is used to store data that can be used and modified throughout your program.

In PHP, variables start with a dollar sign ($).

Syntax

$variable_name = value;

Example

<?php
$name = "Loki";
echo $name;
?>

Output:

Loki

Rules for Naming PHP Variables

PHP variables follow a few important naming rules:

  • A variable must start with a $ sign.
  • The variable name must start with a letter or underscore.
  • A variable name cannot start with a number.
  • Variable names can contain letters, numbers, and underscores.
  • PHP variable names are case-sensitive.

Valid Variable Names

$name = "Loki";
$age = 25;
$user_name = "Ali";
$_email = "example@email.com";

Invalid Variable Names

$1name = "Loki";
$user-name = "Ali";

These variable names are invalid because they either start with a number or contain a hyphen.

Variable Case Sensitivity

PHP variable names are case-sensitive.

<?php
$name = "Loki";
$Name = "Ali";

echo $name;
echo $Name;
?>

Output:

Loki
Ali

These are treated as different variables.

Assigning Values to Variables

String

$name = "Loki";

Integer

$age = 25;

Float

$price = 99.99;

Boolean

$isActive = true;

Array

$colors = ["Red", "Blue", "Green"];

NULL

$name = null;

Combining Variables with Text

Method 1: Concatenation

You can combine strings and variables using the dot (.) operator.

<?php
$name = "Loki";

echo "My name is " . $name;
?>

Output:

My name is Loki

Method 2: Variable Interpolation

PHP also allows you to place variables directly inside double-quoted strings.

<?php
$name = "Loki";

echo "My name is $name";
?>

Output:

My name is Loki

What is a Constant in PHP?

A constant is a value that cannot be changed after it has been defined.

Unlike variables, constants do not use the $ symbol.

Syntax

define("CONSTANT_NAME", "value");

Example

<?php
define("SITE_NAME", "Loki Blog");

echo SITE_NAME;
?>

Output:

Loki Blog

Why Use Constants?

Constants are useful for values that never change.

Examples include:

  • Website name
  • Database configuration
  • API URLs
  • Company information
define("COMPANY_NAME", "ABC Solutions");
define("SUPPORT_EMAIL", "support@example.com");

PHP Data Types

A data type defines what kind of value a variable can store.

PHP supports several data types.

1. String

A string is a sequence of characters.

$name = "Loki";

Example

<?php
echo $name;
?>

2. Integer

An integer is a whole number without decimal points.

$age = 25;

Examples include:

10
25
100
-50

3. Float

A float is a number that contains a decimal point.

$price = 99.99;

Floats are commonly used for prices, measurements, percentages, and other decimal values.

4. Boolean

A Boolean can have only two values:

true
false

Example

$isAdmin = true;
$isPaid = false;

Booleans are commonly used when working with conditions and user permissions.

5. Array

Arrays store multiple values in a single variable.

Example

<?php
$colors = ["Red", "Blue", "Green"];

echo $colors[0];
?>

Output:

Red

Arrays are very important in PHP and are used frequently in WordPress development.

6. NULL

NULL represents a variable with no value.

$name = null;

A variable can contain NULL when no value has been assigned or when its value has intentionally been cleared.

Checking Data Types

PHP provides the var_dump() function to display information about a variable, including its data type.

Example

<?php
$name = "Loki";

var_dump($name);
?>

Output:

string(4) "Loki"

You can use var_dump() while debugging PHP applications to understand the value and type of a variable.

Variables, Constants, and Data Types Together

Let’s combine variables, constants, and different data types in one example.

<?php
$name = "Loki";
$age = 25;
$isPremium = true;

define("SITE_NAME", "Loki Tutorials");

echo "Welcome to " . SITE_NAME;
echo "<br>";
echo "Name: " . $name;
echo "<br>";
echo "Age: " . $age;

?>

Output:

Welcome to Loki Tutorials
Name: Loki
Age: 25

Common Beginner Mistakes

Forgetting the Dollar Sign

Wrong:

name = "Loki";

Correct:

$name = "Loki";

Using Quotes Around Constants

Wrong:

echo "SITE_NAME";

This prints the text SITE_NAME instead of the constant’s value.

Correct:

echo SITE_NAME;

Variable Case Sensitivity

Remember that these are different variables:

$name = "Loki";
$Name = "Ali";

Best Practices for PHP Variables

  • Use meaningful variable names.
  • Use consistent naming conventions.
  • Avoid unnecessary global variables.
  • Initialize variables before using them.
  • Use constants for values that should not change.

Good example:

$user_name = "Loki";
$product_price = 999;
$is_logged_in = true;

Conclusion

PHP variables, constants, and data types are fundamental concepts that every PHP developer needs to understand.

Variables allow you to store and change data, constants allow you to define values that should remain fixed, and data types determine what kind of information a variable contains.

Understanding these concepts will make it much easier to learn PHP operators, conditions, loops, functions, and eventually WordPress development.

In the next tutorial, we’ll learn about PHP Operators and how they are used to perform calculations and make decisions in PHP.