exercism-php/difference-of-squares/DifferenceOfSquares.php

<?php

/*
 * By adding type hints and enabling strict type checking, code can become
 * easier to read, self-documenting and reduce the number of potential bugs.
 * By default, type declarations are non-strict, which means they will attempt
 * to change the original type to match the type specified by the
 * type-declaration.
 *
 * In other words, if you pass a string to a function requiring a float,
 * it will attempt to convert the string value to a float.
 *
 * To enable strict mode, a single declare directive must be placed at the top
 * of the file.
 * This means that the strictness of typing is configured on a per-file basis.
 * This directive not only affects the type declarations of parameters, but also
 * a function's return type.
 *
 * For more info review the Concept on strict type checking in the PHP track
 * <link>.
 *
 * To disable strict typing, comment out the directive below.
 */

declare(strict_types=1);

function squareOfSum(int $max): int {
  $res = $max * ($max + 1) / 2;
  return $res * $res;
}

function sumOfSquares(int $max): int {
  $sum = 0;
  for ($i = 1; $i <= $max; $i++) {
    $sum += $i * $i;
  }
  return $sum;
}

function difference(int $max): int {
  return squareOfSum($max) - sumOfSquares($max);
}