What exactly does this mean?
$number = ( 3 - 2 + 7 ) % 7;
It's the modulus operator, as mentioned, which returns the remainder of a division operation.
Examples: 3%5
returns 3, as 3 divided by 5 is 0 with a remainder of 3.
5 % 10
returns 5, for the same reason, 10 goes into 5 zero times with a remainder of 5.
10 % 5
returns 0, as 10 divided by 5 goes exactly 2 times with no remainder.
In the example you posted, (3 - 2 + 7)
works out to 8, giving you 8 % 7
, so $number
will be 1
, which is the remainder of 8/7.