I keep getting an the following error of Array to string conversion error on this line listed below. How can I fix this problem?

$skill = explode('', $_POST['skill']);

Here is the PHP & MySQL code.

$skill = explode('', $_POST['skill']);
$experience = explode('', $_POST['experience']);
$years = explode('', $_POST['years']);

for ($s = 0; $s < count($skill); $s++){
    for ($x = 0; $x < count($experience); $x++){
        for ($g = 0; $g < count($years); $g++){

            if (mysqli_num_rows($dbc) == 0) {
                    $mysqli = mysqli_connect("localhost", "root", "", "sitename");
                    $dbc = mysqli_query($mysqli,"INSERT INTO learned_skills (user_id, skill, experience, years, date_created) 
                                                 VALUES ('" . $user_id . "', '" . $skill[$s] . "', '" . $experience[$x] . "', '" . $years[$g] . "', NOW())");
            }

            if ($dbc == TRUE) {
                    $dbc = mysqli_query($mysqli,"UPDATE learned_skills 
                                                 SET skill = '$skill', experience = '$experience', years = '$years', date_created = NOW()
                                                 WHERE user_id = '$user_id'");

                    echo '<p class="changes-saved">Your changes have been saved!</p>';
            }

            if (!$dbc) {
                    print mysqli_error($mysqli);
                    return;
            }

        }
    }
}