Just like most languages you can use PHP to do Arithmetic equations like so:

echo (10 / 2);//5
echo (10 + 2);//12
echo (10 * 2);//20
echo (10 - 2);//8
echo (10 % 2);//0

$value = 50;
$value2 = 10;
echo ($value * $value2);//500
echo ($value - ($value + $value2));//10

To check if a value or variable is even use:

if (($var % 2) == 1)
{ echo "$var is odd." ;}
if (($a % 2) == 0)
{ echo "$var is even." ;}

To alternate a script output:

for ($i = 1; $i <= 10; $i++) {
  if(($i % 2) == 1)
    {echo "<h1>$i is Even</h1>";}
  else
    {echo "<h1>$i is Odd</h1>";}
 }