What does the percent sign mean in PHP?

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.

It is the modulus operator:

$a % $b = Remainder of $a divided by $b.

It is often used to get "one element every N elements". For instance, to only get one element each three elements:

for ($i=0 ; $i<10 ; $i++) {
    if ($i % 3 === 0) {
        echo $i . '<br />';
    }
}

Which gets this output:

0
3
6
9

(Yeah, OK, $i+=3 would have done the trick; but this was just a demo.)