The specific error you're getting is because this:
elseif ((5/9) * ($num1 - 32)
Needs to be:
elseif ((5/9) * ($num1 - 32))
(Missing closing paren)
I think your entire script could be rewritten like this:
for ($num=10; $num <= 100; $num += 10) {
$f = (($num * 9) / 5) + 32;
echo "$num converted to fahrenheit is $f</br>";
}
Working example: http://3v4l.org/iT7p0