<?php
$answer = 0;
// 20*20 の格子を配列にする
$grid = array(array(08,02,22,97,38,15,00,40,00,75,04,05,07,78,52,12,50,77,91,08),
array(49,49,99,40,17,81,18,57,60,87,17,40,98,43,69,48,04,56,62,00),
array(81,49,31,73,55,79,14,29,93,71,40,67,53,88,30,03,49,13,36,65),
array(52,70,95,23,04,60,11,42,69,24,68,56,01,32,56,71,37,02,36,91),
array(22,31,16,71,51,67,63,89,41,92,36,54,22,40,40,28,66,33,13,80),
array(24,47,32,60,99,03,45,02,44,75,33,53,78,36,84,20,35,17,12,50),
array(32,98,81,28,64,23,67,10,26,38,40,67,59,54,70,66,18,38,64,70),
array(67,26,20,68,02,62,12,20,95,63,94,39,63,08,40,91,66,49,94,21),
array(24,55,58,05,66,73,99,26,97,17,78,78,96,83,14,88,34,89,63,72),
array(21,36,23,09,75,00,76,44,20,45,35,14,00,61,33,97,34,31,33,95),
array(78,17,53,28,22,75,31,67,15,94,03,80,04,62,16,14,09,53,56,92),
array(16,39,05,42,96,35,31,47,55,58,88,24,00,17,54,24,36,29,85,57),
array(86,56,00,48,35,71,89,07,05,44,44,37,44,60,21,58,51,54,17,58),
array(19,80,81,68,05,94,47,69,28,73,92,13,86,52,17,77,04,89,55,40),
array(04,52,08,83,97,35,99,16,07,97,57,32,16,26,26,79,33,27,98,66),
array(88,36,68,87,57,62,20,72,03,46,33,67,46,55,12,32,63,93,53,69),
array(04,42,16,73,38,25,39,11,24,94,72,18,08,46,29,32,40,62,76,36),
array(20,69,36,41,72,30,23,88,34,62,99,69,82,67,59,85,74,04,36,16),
array(20,73,35,29,78,31,90,01,74,31,49,71,48,86,81,16,23,57,05,54),
array(01,70,54,71,83,51,54,69,16,92,33,48,61,43,52,01,89,19,67,48));
for($i = 0 ; $i < 20 ; $i++)
{
for($j = 0 ; $j < 20 ; $j++)
{
// 縦
if($j < 17 and $answer < $grid[$i][$j] * $grid[$i][$j + 1 ] * $grid[$i][$j + 2] * $grid[$i][$j + 3])
{
$answer = $grid[$i][$j] * $grid[$i][$j + 1 ] * $grid[$i][$j + 2] * $grid[$i][$j + 3];
}
// 横
if($i < 17 and $answer < $grid[$i][$j] * $grid[$i + 1][$j] * $grid[$i + 2 ][$j] * $grid[$i + 3][$j])
{
$answer = $grid[$i][$j] * $grid[$i + 1][$j] * $grid[$i + 2 ][$j] * $grid[$i + 3][$j];
}
// 斜め右下
if($i < 17 and $j < 17 and $answer < $grid[$i][$j] * $grid[$i + 1][$j + 1] * $grid[$i + 2 ][$j + 2] * $grid[$i + 3][$j + 3])
{
$answer = $grid[$i][$j] * $grid[$i + 1][$j + 1] * $grid[$i + 2 ][$j + 2] * $grid[$i + 3][$j+ 3];
}
// 斜め左下
if($i >= 3 and $j < 17 and $answer < $grid[$i][$j] * $grid[$i - 1][$j + 1] * $grid[$i - 2 ][$j + 2] * $grid[$i - 3][$j + 3])
{
$answer = $grid[$i][$j] * $grid[$i - 1][$j + 1] * $grid[$i - 2 ][$j + 2] * $grid[$i - 3][$j+ 3];
}
}
}
echo $answer;
?>