What are the differences between PHP constants and variables ?

PHP Constants: PHP Constants are the identifiers that remain the same. Usually, it does not change during the execution of the script. They are case-sensitive. By default, constant identifiers are always uppercase. Usually, a Constant name starts with an underscore or a letter which is followed by a number of letters and numbers. They are no need to write a constant with the $ sign. The constant() function is used to return the value of a constant.

Example:

<?php
   define("Hello", "Welcome to geeksforgeeks.com!");
   echo Hello;
?>

Output:

Welcome to geeksforgeeks.com!

PHP Variables: A PHP variable is a name given to a memory address that holds data. The basic method to declare a PHP variable is by using a $ sign which is followed by the variable name. A variable helps the PHP code to store information in the middle of the program. If we use a variable before it is assigned then it already has a default value stored in it. Some of the data types that we can use to construct a variable are integers, doubles, and boolean.

Example:

<?php
   $txt = "Hello from geeksforgeeks!";
   $x = 5;
   $y = 10.5;
   echo $txt;
   echo "<br>";
   echo $x;
   echo "<br>";
   echo $y;
?>

Output:

Hello from geeksforgeeks!
5
10.5

Difference between PHP Constants and PHP Variables