<?php
function calculate(array $in)
{
    foreach ($in as $i => $h) {
        $m[$i] = max(min(array_reduce(array_slice($in, 0, $i), 'max'), array_reduce(array_slice($in, $i), 'max')) - $h, 0);
    }
    return array_sum($m);
}


print_r(calculate(array(0, 5, 0, 1, 0, 2, 0))); echo "\n";
print_r(calculate(array(2, 5, 1, 2, 3, 4, 7, 7, 6)));