// return true if array is associative
function checkAssoc($array) {
    $nonAssociative = count(array_filter(array_keys($array), 'is_string')) === 0;
    if ($nonAssociative) {
        return false;
    } else {
        return true;
    }
}
// Example
$array = ["el1" => 1, "el2" => 2, "el3" => 3];
checkAssoc($array);
// bool(true)