Numbered basics chapters for sortability

chapters/basics/03-arithmetic.md

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+Now that we know how to create variables, let's look at doing some math.
+```php
+<?php
+
+$a = 1;
+$b = 2;
+```
+
+To add two values, use the plus symbol.
+```php
+echo $a + $b;
+```
+
+To subtract, use the minus symbol.
+```php
+echo $b - $a;
+```
+
+To multiply two values, use an asterisk.
+```php
+echo $a * $b;
+```
+
+To divide values, use a forward slash.
+```php
+echo $b / $a;
+```
+
+PHP uses the percent symbol for calculating the modulus of two numbers.
+The modulus is calculated by dividing two numbers and returning the remainder of the result.
+In this example, the value of $mod will be 0.
+```php
+$mod = 10 % 5;
+```
+
+You can also use double asterisks to calculate a number to the power of another number.
+The following statement will print 25.
+```php
+echo 5 ** 2;
+```