file.php

//Problem: Given an array of integers, find the pair of adjacent elements that has the largest product and return that product.

//[-23, 4, -3, 8, -12]
// -23 * 4 = -92
// 4 * -3 = -12
// -3 * 8 = -24
// 8 * -12 = -96
// Return -12

//Loop N-1 times starting at pos 0

function adjacentElementsProduct(array $inputArray) { 
  $largestTotal = null;
  foreach ($inputArray as $key => $value) { //You loop over all elements. You should skip the last element.
    $nextValue = $inputArray[$key + 1]; // <--- Undefined Offset at N-1
    $total = $value * $nextValue; // $value * null == 0
    if (!isset($largestTotal) || $total > $largestTotal) {  // false || 0 > -12 ==> True
      $largestTotal = $total;  //0
    } 
  } 
  return $largestTotal;  //0
}