PHP constants are named values that cannot be changed once they are defined. They are defined using the define() function and are used to store values that are not expected to change, such as configuration options or mathematical constants. They are also case-sensitive, which means that MY_CONSTANT is different from my_constant.

On the other hand, variables are named storage locations that can hold different values at different times during the execution of a program. They are defined using the $ symbol and are used to store values that are expected to change, such as user input or data from a database. They are case-sensitive, just like constants.

One key difference between constants and variables is that constants cannot be modified once they are defined, whereas variables can be modified at any time during the execution of a program. This means that you cannot use the = operator to assign a new value to a constant, but you can use it to assign a new value to a variable.

Here’s an example that demonstrates how to define and use constants and variables in PHP:

<?php

// Define a constant
define('MY_CONSTANT', 'Hello, world!');

// Define a variable
$myVariable = 'Goodbye, world!';

// Print the constant
echo MY_CONSTANT; // Output: "Hello, world!"

// Print the variable
echo $myVariable; // Output: "Goodbye, world!"

// Try to modify the constant (this will produce an error)
MY_CONSTANT = 'Foo';

// Modify the variable
$myVariable = 'Bar';

// Print the modified variable
echo $myVariable; // Output: "Bar"

I hope this helps! Let me know if you have any questions or need further clarification.