aqi.php

<?php

function aqiFromPM($pm) {
  if($pm >= 350.5)
    return scale($pm, 500, 401, 500, 350.5);
  if($pm >= 250.5)
    return scale($pm, 400, 301, 350.4, 250.5);
  if($pm >= 150.5)
    return scale($pm, 300, 201, 250.4, 150.5);
  if($pm >= 55.5)
    return scale($pm, 200, 151, 150.4, 55.5);
  if($pm >= 35.5)
    return scale($pm, 150, 101, 55.4, 35.5);
  if($pm >= 12.1)
    return scale($pm, 100, 51, 35.4, 12.1);
  if($pm >= 0)
    return scale($pm, 50, 0, 12, 0);
  return '';
}

function scale($pm, $aqiHigh, $aqiLow, $pmHigh, $pmLow) {
  $a = $aqiHigh - $aqiLow;
  $b = $pmHigh - $pmLow;
  $c = $pm - $pmLow;
  return round(($a/$b) * $c + $aqiLow);
}