https://www.hackerrank.com/challenges/diagonal-difference/problem

diagonal_matrix_sum.php

<?php

$handle = fopen('php://stdin', 'r');
fscanf($handle, '%d', $n);
$a = [];

for ($i = 0; $i < $n; $i++) {
	$temp = fgets($handle);
	$a[] = explode(' ', $temp);
	array_walk($a[$i], 'invtal');
}

$d1 = $d2 = $j = 0;
$length = count($a);

for ($i = 0; $i < $length; $i++) {
	$d1 += $a[$j++][$i];
	$d2 += $a[$length-$j][$i];
}

print abs($d1 - $d2);