function get_non_zero( $str_keys, $data ) {
    $keys = explode( ',', $str_keys );
    foreach ( $keys as $key ) {
        if ( ! empty( $data[ $key ] ) ) {
            return $data[ $key ];
        }
    }

    return null;
}

$data = ['key0' => 0, 'key1' => 1, 'key3'=> 3];
echo( get_non_zero( 'key1,key2', $data ) ); // выведет 1

$data = ["key0" => 0, "key1" => 1, "key3"=> 3];

$search = array_flip(explode(",", "key1,key2"));
$result = array_intersect_key($data, $search);

echo count($result) > 1 ? null : array_pop($result);

function getNonZero($keys, $data)
{
    $search = array_flip(explode(",", $keys));
    $result = array_intersect_key($data, $search);

    if (count($result) > 1):
        return null;
    else:
        return array_pop($result);
    endif;
}

$data = ["key0" => 0, "key1" => 1, "key3"=> 3];
$search = "key1,key2";

echo getNonZero($search, $data);

function searchInArray($value)
{
    preg_match("([w+]{1,})", $value, $matches);
    return !empty($matches) ? $matches[0] : $value;
}

function getNonZero($search, $data)
{
    $search = array_map("searchInArray", explode(",", $search));
    $result = array_intersect_key($data, array_flip($search));

    if (count($result) === 1):
        return is_array($result) ? array_shift($result[key($result)]) : $result;
    else:
        return null;
    endif;
}

$data = ['key0' => 0, 'fields' => ['name_1'=>1], 'key3'=> 3];
$sting = 'name,entry.436917433,billing_first_name,fields[name_1]';

echo getNonZero($sting, $data);