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PHP Numbers
Mathematical Assignment Operators

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:

Operation: Long Syntax: Short Syntax:
Add \$x = \$x + \$y \$x += \$y
Subtract \$x = \$x - \$y \$x -= \$y
Multiply \$x = \$x * \$y \$x *= \$y
Divide \$x = \$x / \$y \$x /= \$y
Mod \$x = \$x % \$y \$x %= \$y

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!

### Instructions

1.

We’re going to do a mathematical “magic” trick. Create a variable, `\$my_num` and assign as its value any number.

Next, create a second variable, `\$answer` and assign `\$my_num` as its value.

2.

Use the addition assignment operator to add 2 to `\$answer`.

3.

Use the multiplication assignment operator to multiply `\$answer` by 2.

4.

Use the subtraction assignment operator to subtract 2 from `\$answer`.

5.

Use the division assignment operator to divide `\$answer` by 2.

6.

Almost there. Use the subtraction assignment operator to subtract your original number (`\$my_num`) from `\$answer`. Finally, use `echo` to print `\$answer` to the terminal.

If everything went as it should, `\$answer` should be 1. No matter what your original number was! Don’t believe us? Go ahead try it with a different number for the value of `\$my_num`.