One common task when manipulating number variables is to reassign them to their old value with some operation performed on it.

$savings = 800;
$bike_cost = 75;
$savings = $savings - $bike_cost;
echo $savings; // Prints: 725

This is such a common task that PHP provides a shorter syntax using arithmetic assignment operators:

We could use this shorter syntax to rewrite the above code:

$savings = 800;
$bike_cost = 75;
$savings -= $bike_cost;
echo $savings; // Prints: 725

With mathematical assignment operators, PHP doesn’t allow spaces between the two characters.

Ready for one more shortcut? Increment operators allow us to subtract or add one to a number with just two characters.

$age = 89; 
$age++;
echo $age; // Prints: 90

$days_til_vacation = 7; 
$days_til_vacation--;
echo $days_til_vacation; // Prints: 6

In the code above, we used the post-increment (++) operator to add one to $age and we used the post-decrement operator (--) to subtract one from $days_til_vacation.

Those keystrokes can add up, so let’s practice using arithmetic assignment operators on variables!