<?php

$black = true;
$i = 0;
while($i < 20){
	if(($i % 2) == 0){//if the number is even
		
		$black = !$black;  //

		if($black){
			$class = 'blackRow';
		}else{
			$class = 'whiteRow';
		}
		
	}  
	echo $i . " - " ;
	echo $class  . "\r\n";
	$i++;	
}

0 - whiteRow
1 - whiteRow
2 - blackRow
3 - blackRow
4 - whiteRow
5 - whiteRow
6 - blackRow
7 - blackRow
8 - whiteRow
9 - whiteRow
10 - blackRow
11 - blackRow
12 - whiteRow
13 - whiteRow
14 - blackRow
15 - blackRow
16 - whiteRow
17 - whiteRow
18 - blackRow
19 - blackRow