Codility Equi

function solution($A) {
    $itemsSum = array_sum($A);

    $leftSum = 0;
    $rightSum = $itemsSum;

    for ($equilibriumIndex = 0, $count = count($A); $equilibriumIndex < $count; ++$equilibriumIndex) {
        if ($equilibriumIndex > 0) {
            $leftSum += $A[$equilibriumIndex - 1];
        }

        $rightSum -= $A[$equilibriumIndex];

        if ($leftSum === $rightSum) {
            return $equilibriumIndex;
        }
    }

    return -1;
}