PHP mysqli reusable functions for SELECT and INSERT/UPDATE queries [closed]

I’m learning PHP and wanted to create two simple reusable functions for MySQL database access —
one for selecting data, and one for modifying data.

I’m not sure if this is a good and safe way to handle it.
Can this structure cause any issues (for example: performance, error handling, or security problems)?

Here’s my code:

<?php
function getData($sql) {
    $db = new mysqli("localhost", "root", "", "cars");
    if ($db->connect_errno != 0) {
        return $db->connect_errno;
    }

    $result = $db->query($sql);
    if ($db->errno != 0) {
        return $db->error;
    }

    if ($result->num_rows == 0) {
        return [];
    }

    return $result->fetch_all(MYSQLI_ASSOC);
}

function setData($sql) {
    $db = new mysqli("localhost", "root", "", "cars");
    if ($db->connect_errno != 0) {
        return $db->connect_errno;
    }

    $db->query($sql);
    if ($db->errno != 0) {
        return $db->error;
    }

    return $db->affected_rows > 0 ? true : false;
}
?>

I expected these two functions to simplify my database access:

• adatel() should return all selected rows as an array
• adatVal() should return true/false depending on whether the query affected rows

It seems to work, but I want to make sure this is a good approach.
Should I use prepared statements instead, or is this fine for small school projects?