Variables keep a value for future reference. This value can change if we want it to; that is why they are called variables. Let's take a look at them in an example. Save this code in your index.php file:

<?php
$a = 1;
$b = 2;
$c = $a + $b;
echo $c; // 3

In this preceding piece of code, we have three variables: $a has value 1, $b has 2, and $c contains the sum of $a and $b, hence, $c equals 3. Your browser should print the value of the variable $c, which is 3.

Assigning a value to a variable means to give it a value, and it is done with the equals sign as shown in the previous example. If you did not assign a value to a variable, we will get a notice from PHP when it checks its contents. A notice is just a message telling us that something is not exactly right, but it is a minor problem and you can continue with the execution. The value of an unassigned variable will be null, that is, nothing.

PHP variables start with the $ sign followed by the variable name. A valid variable name...