findDeletedNumber.php

<?php
// An ordered sequence of numbers from 1 to N is given. One number might have deleted from it, then the remaining numbers were mixed. Find the number that was deleted.
//
// Example:
//
// The starting array sequence is [1,2,3,4,5,6,7,8,9]
// The mixed array with one deleted number is [3,2,4,6,7,8,1,9]
// Your function should return the int 5.
// If no number was deleted from the array and no difference with it, your function should return the int 0.
//
// Note that N may be 1 or less (in the latter case, the first array will be []).

function findDeletedNumber(array $arr, array $mixedArr): int {
  return  count($arr) === count($mixedArr) ? 0 : array_values(array_diff($arr,$mixedArr))[0];
}

// Alternative Solution:
// function findDeletedNumber(array $arr, array $mixedArr): int {
//   return array_sum($arr) - array_sum($mixedArr);
// }


?>