To do simple math in PHP you use operators including + - * / % ++ --

Use the + character to add
Use the - character to subtract
Use the * to multiple
Use the / to divide
Use the % to get just the remainder of you operation
Use the ++ to go UP in an incuments of 1
Use the -- to go DOWN in an incuments of 1

In order to see these math operation you set a varible to the product of your math problem then you can call echo that variable to print

Addition: 10 + 10
20

Subtraction: 15 - 5
10

Multiplication: 5 * 5
25

Division: 15/5
3

Modulous: Remainder of 8/5
3

Up 1 increment of ++10
Increments 10 up by 1, then returns 11.
After it adds 1 it return the new number
11

Up 1 increment: 10++
Returns 10, then increments 1 up to 11.
It returns the current number then adds 1 after
10

DOWN 1 increment of --10
Increments 10 down by 1, then returns 9.
After it subtracts 1 it return the new number
9

DOWN 1 increment: 10--
Returns 10, then increments down by 1 to 9.
It returns the current number then subtracts 1
10