The modulo needed to target specific elements would be 10 == 0, since you want all the tens.

echo $count % 10 == 0 ? $count : '<b>' . $count . '</b>', ' ';

Though I'd have to know what the data is and WHY it's getting bold or why it isn't... since data dictates what the actual markup should be. From an efficiency standpoint in terms of what's created for markup, the above is trash.

In any case, a modulo is just the remainder after a divide... so %2 means that 0 is 0, 1 is 1, 2 is 0, 3 is 1, and so forth. %10 means that you just get the final digit, 0..9 as the result...

Code:

0 / 10 == 0 remainder 0, so 0 % 10 == 0
2 / 10 == 0 remainder 2, so 2 % 10 == 2
28 / 10 == 2 remainder 8, so 28 % 10 == 8
30 / 10 == 3 remainder 0, so 30 % 10 == 0

That's all it does, is give you the remainder... so to target all multiples of ten, you $count % 10 == 0

*Interstingly under the hood on x86 platforms the operation for divide and modulo are the same -- divide. It returns the integer result in AX and remainder in DX. This can lead to many C and other high level languages doing divide (a slow operation) multiple times when it doesn't have to.*