<?php

$arr1 = [1, 2, 3, 4];
$arr2 = [3, 2, 2, 1, 0, 6, 1];
$arr3 = [1, 3, 2, 1, 5, 4, 1];

function fun($arr, $acc = 0) {
  if(max($arr) == null) {
     return $acc;

  }  else {
    $acc = $acc + max($arr);
    $key = array_search(max($arr), $arr);
    if ($key === 0) {
        $arr[$key] = null;
        $arr[$key + 1] = null;
        return fun($arr, $acc);

    } elseif ($key === (count($arr) - 1)) {
      $arr[$key] = null;
      $arr[$key - 1] = null;
      return fun($arr, $acc);

    } else {
      $arr[$key] = null;
      $arr[$key + 1] = null;
      $arr[$key - 1] = null;
      return fun($arr, $acc);

    }
  }
}

function rob($ar){
  return fun($ar);
}


echo rob($arr1)."\n";
echo rob($arr2)."\n";
echo rob($arr3)."\n";