bmi.php

<?php namespace BodyMassIndex;

abstract class Thresholds {
  const UNDERWEIGHT = 18.5;
  const OVERWEIGHT = 25;
  const OBESE = 30;
}

/**
 * Calculates Body Mass Index.
 * @param float $mass body mass [kg]
 * @param float $height body height [m]
 * @return float Returns calculated Body Mass Index [kg/m^2]
 */
function calc($mass, $height) {
  return $mass / pow($height, 2);
}

/**
 * Gets BMI category.
 * @param float $bmi Body Mass Index
 * @return string Returns BMI category
 */
function category($bmi) {
  if ($bmi > Thresholds::OBESE) return "obese";
  elseif ($bmi > Thresholds::OVERWEIGHT) return "overweight";
  elseif ($bmi < Thresholds::UNDERWEIGHT) return "underweight";
  else return "normal";
}

foo.php

<?php 

include 'bmi.php';
use BodyMassIndex as bmi;

$bmi = bmi\calc(48, 1.59);
print bmi\category($bmi) . '\n';