多个SQL插入（插入次数取决于用户输入的数量）

I have a module on my website which allows an admin to add some users to a group.

It is build like that (with selectboxes), you can select multiple users but only one group:

Users:                       Group:
- user 1                     - group 1
- user 2                     - group 2
- user 3
- user 4
- user 5

Inside the PHP, it looks like this:

$user_id = $_POST["user_id"]; //this can be an array
$group_id = $_POST["group_id"];

if (count($user_id) == 1) { 
    $sql = "INSERT IGNORE INTO users_groups (user_id, group_id) VALUES ($user_id[0],$group_id)"; 
} elseif (count($user_id) == 2) {
    $sql = "INSERT IGNORE INTO users_groups (user_id, group_id) VALUES ($user_id[0],$group_id), ($user_id[1],$group_id)"; 
} elseif (count($user_id) == 3) {
    $sql = "INSERT IGNORE INTO users_groups (user_id, group_id) VALUES ($user_id[0],$group_id), ($user_id[1],$group_id), ($user_id[2],$group_id)"; 
} elseif (count($user_id) == 4) {
    $sql = "INSERT IGNORE INTO users_groups (user_id, group_id) VALUES ($user_id[0],$group_id), ($user_id[1],$group_id), ($user_id[2],$group_id), ($user_id[3],$group_id)"; 
} elseif (count($user_id) == 5) {
    $sql = "INSERT IGNORE INTO users_groups (user_id, group_id) VALUES ($user_id[0],$group_id), ($user_id[1],$group_id), ($user_id[2],$group_id), ($user_id[3],$group_id), ($user_id[4],$group_id)"; 
} else {
    echo'Error: Too many users selected';
    exit;
}

How can I get this in a shorter code? Maybe with prepared statements because the inputs are from users.

Are you looking for something like this?

if(count($user_id)>5){
    echo 'Error: Too many users selected';
    exit;
}
$sql = "INSERT IGNORE INTO users_groups (user_id, group_id) VALUES ($user_id[0],$group_id)";
for ( $i = 1; $i < count($user_id); $i++) {
    $sql .= ", ($user_id[$i],$group_id)"
}