How to calculate the sum of every possible combinations of the Integer array?

Given an array of positive integers a, I want to calculate the sum of every possible a[i] ∘ a[j], where a[i] ∘ a[j] is the concatenation of the string representations of a[i] and a[j] respectively.

Example

For a = [10, 2], the output should be solution(a) = 1344.

a[0] ∘ a[0] = 10 ∘ 10 = 1010,
a[0] ∘ a[1] = 10 ∘ 2 = 102,
a[1] ∘ a[0] = 2 ∘ 10 = 210,
a[1] ∘ a[1] = 2 ∘ 2 = 22.
So the sum is equal to 1010 + 102 + 210 + 22 = 1344.

For a = [8], the output should be solution(a) = 88.

For a = [1, 2, 3], the output should be solution(a) = 198.

a[0] ∘ a[0] = 1 ∘ 1 = 11,
a[0] ∘ a[1] = 1 ∘ 2 = 12,
a[0] ∘ a[2] = 1 ∘ 3 = 13,
a[1] ∘ a[0] = 2 ∘ 1 = 21,
a[1] ∘ a[1] = 2 ∘ 2 = 22,
a[1] ∘ a[2] = 2 ∘ 3 = 23,
a[2] ∘ a[0] = 3 ∘ 1 = 31,
a[2] ∘ a[1] = 3 ∘ 2 = 32,
a[2] ∘ a[2] = 3 ∘ 3 = 33.
The total result is 11 + 12 + 13 + 21 + 22 + 23 + 31 + 32 + 33 = 198.

Answer

The above task can be solved using the following PHP code:

Notes: $a = Integer array, $result = Integer

function solution($a) {
    $result = 0;
    for($i=0;$i<count($a);$i++) {
        for($j=0;$j<count($a);$j++) {
            $result += (int)$a[$i].$a[$j];
        }
    }
    return $result;
}

Try my above solution.