Variables and Constants in php

Jun 20, 2026Trushi Jasani

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Variables are containers for storing data values. Constants are identifiers for simple values that cannot be changed during the execution of a script.

php Variables

In php, a variable starts with the $ sign, followed by the name of the variable:

<?php
$name = "John";
$age = 25;
$height = 5.9;
echo $name;   // John
echo $age;    // 25
echo $height; // 5.9
?>

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php Variable Rules

• A variable starts with the $ sign, followed by the name
• A variable name must start with a letter or the underscore character
• A variable name cannot start with a number
• A variable name can only contain alpha-numeric characters and underscores (A–z, 0–9, and _)
• Variable names are case-sensitive ($age and $Age are different variables)

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Assigning Variables

php is a loosely typed language — you do not need to declare a variable's data type explicitly:

<?php
$txt = "Hello World";   // string
$x = 5;                 // integer
$y = 10.5;              // float
$isValid = true;        // boolean
?>

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Variable Scope

php has three types of variable scope:

• Local – declared inside a function, accessible only within it
• Global – declared outside a function, accessible only outside it (use global keyword inside functions)
• Static – retains its value after the function execution

<?php
$globalVar = "I am global";

function testScope() {
    global $globalVar;
    echo $globalVar; // accessible now
}

function testStatic() {
    static $count = 0;
    $count++;
    echo $count;
}

testStatic(); // 1
testStatic(); // 2
testStatic(); // 3
?>

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php Constants

A constant is an identifier for a simple value. The value cannot be changed during the script. A valid constant name starts with a letter or underscore (no $ sign).

Using define()

<?php
define("SITE_NAME", "MyWebsite");
define("MAX_SIZE", 100);

echo SITE_NAME; // MyWebsite
echo MAX_SIZE;  // 100
?>

Using const keyword

<?php
const VERSION = "1.0.0";
const PI = 3.14159;

echo VERSION; // 1.0.0
echo PI;      // 3.14159
?>

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Constants vs Variables

Feature	Variable	Constant
Prefix	$ sign required	No $ sign
Value change	Can be changed	Cannot be changed
Scope	Limited (local/global)	Global by default
Declaration	Assignment	define() or const

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Predefined Constants

php has several built-in constants available at all times:

<?php
echo PHP_VERSION;    // e.g., 8.2.0
echo PHP_INT_MAX;    // maximum integer value
echo PHP_EOL;        // end-of-line character
echo __FILE__;       // full path and filename of the file
echo __LINE__;       // current line number
?>

Note: Constants are automatically global and can be used across the entire script.