<?php

//Sample application to find available values ​​in a merged collection:

function checkVacancies(array $positions) {

    $available = [];

foreach ($positions as $position => $optionAvailable) {
 
       $nextBox = false;
        $prevBox = false;

        if (isset($positions[$position+1])) {
           $nextBox = $positions[$position+1];
        }
        if (isset($positions[$position-1])) {
           $prevBox = $positions[$position-1];
        }

        if ($nextBox == false && $prevBox == false && $optionAvailable == false)  {
             $available[] = $position;
         }

  }
     return  $available;
}
//false is returned position only if there is a real approximate value


$positions = [
true, //0
false,
false, //2
false, 
true,
false, 
false, //8
false, //
false
];

print_r(checkVacancies($positions));