Solution in PHP

function diagonalDifference(array arraySize = count($arr);

    // primary & secondary sum vars
    `$primarySum = $`secondarySum = 0;

    // looping the array
    for (`$x = 0; $`x < `$arraySize; $`x++) {
        // incrementing primary sum 
        `$primarySum += $`arr[`$x][$`x];

        // incrementing secondary sum
        `$secondarySum += $`arr[`$x][$`arraySize - 1 - $x];
    }

    // return absolute diagonal
    return abs(`$primarySum - $`secondarySum);

};