PHP – variables, data types, constants, and syntax basics

In this chapter, you will learn the fundamental elements of PHP, which are the foundation for creating dynamic web applications. We will cover variables, data types, constants, variable scope, and the difference between single quotes and double quotes. You will also learn how concatenation works in PHP.

Variables in PHP

Variables in PHP start with the $ sign. They can store different values and change their type during program execution.

<?php
$name = "John";
$age = 25;
$isActive = true;

Data types

• string – text
• int – integers
• float – floating-point numbers
• bool – boolean values (true/false)
• array – arrays
• object – objects
• null – no value

Constants in PHP

Constants are values that cannot be changed during program execution. We create them using define() or const.

<?php
define("SITE_NAME", "MyWebsite");
const VERSION = "1.0";

Variable scope

Variables can have different scopes:

• Global – available throughout the script
• Local – available only inside a function
• Static – remember their value between function calls

<?php
function counter() {
    static $i = 0;
    $i++;
    return $i;
}

echo counter(); // 1
echo counter(); // 2

Single quotes vs. double quotes

In PHP, there is a difference between using single quotes (' ') and double quotes (" "):

• Single quotes: the text is interpreted literally.
• Double quotes: allow variable interpolation.

<?php
$name = "John";
echo 'Hello $name'; // Hello $name
echo "Hello $name"; // Hello John

Concatenation

Concatenation means joining strings using the . operator.

<?php
$firstName = "John";
$lastName = "Smith";
echo $firstName . " " . $lastName; // John Smith

Best practices

1. Name variables and constants descriptively ($userAge, MAX_USERS).
2. Use const instead of define() when defining constants in application code.
3. Avoid mixing single and double quotes unnecessarily.