Note that int always rounds towards zero. To get it to round (more) correctly:

$newtemp =100*((($temp-32)*5)/9);
if ($newtemp > 0) {
  $newtemp = int($newtemp + 0.5)/100;
} else {
  $newtemp = int($newtemp - 0.5)/100;
}
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But (s)printf really is the right solution here...