Q: HTML Form Data to SQL Database Information
I made a PHP script that connects to my database and creates a record in my database. This works. Now, I added a simple form to this page and I want to make a record in my database based on the input that a user gives.
The PHP script without the form:
<?php
$servername = "localhost";
$username = "root";
$password = "";
$dbname = "detachering";

// Create connection
$conn = new mysqli($servername, $username, $password, $dbname);
// Check connection
if ($conn->connect_error) {
    die("Connection failed: " . $conn->connect_error);
}

$sql = "INSERT INTO medewerkers (voornaam, achternaam, leeftijd, uurloon, opleidingsniveau)
VALUES ('test', 'user', '20', '192', 'HBO')";

if ($conn->query($sql) === TRUE) {
    echo "Medewerker is aangemaakt";
} else {
    echo "Error: " . $sql . "<br>" . $conn->error;
}

$conn->close();
?>

This is the form that I want to add to it:
<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8">
    <title></title>
  </head>
  <body>
      <form class="" action="index.html" method="post">
        <input type="text" name="voornaam" placeholder="Voornaam">
        <input type="text" name="achternaam" placeholder="Achternaam">
        <input type="text" name="leeftijd" placeholder="Leeftijd">
        <input type="text" name="uurloon" placeholder="Uurloon">
        <input type="text" name="opleidingsniveau" placeholder="Opleidingsniveau">
        <button type="submit" name="button">Save</button>
      </form>
  </body>
</html>

I personally think that it should be something like:
INSERT INTO medewerkers (voornaam, achternaam, leeftijd, uurloon, opleidingsniveau)
VALUES ('POST_['voornaam']', 'POST_['achternaam']', 'POST_['leeftijd']', 'POST_['uurloon']', 'POST_['opleidingsniveau']');

I haven't done much in PHP, so any help is greatly appreciated!

A: Use prepared statements for this.
Unfortunately, with mysqli you'll have to use references and cannot just use the POST array directly.
$stmt = $conn->prepare('INSERT INTO medewerkers (voornaam, achternaam, leeftijd, uurloon, opleidingsniveau) VALUES(?, ?, ?, ?, ?)');
if ($stmt) {
    $voornaam = $_POST['voornaam'];
    $achternaam = $_POST['achternaam'];
    $leeftijd = $_POST['leeftijd']
    $uurloon = $_POST['uurloon'];
    $opleidingsniveau = $_POST['opleidingsniveau'];

    $stmt->bind_param('sssss', $voornaam, $achternaam, $leeftijd, $uurloon, $opleidingsniveau);
    $stmt->execute();
}

edit: Since we are talking about it, here is the PDO example (with connection):
try {
    $dsn = 'mysql:host=localhost;dbname=test';
    $pdo = new PDO($dsn, 'root', 'passwd');
    $pdo->exec('SET CHARACTER SET UTF8');
    $pdo->setAttribute(PDO::ATTR_DEFAULT_FETCH_MODE, PDO::FETCH_ASSOC);
    $pdo->setAttribute( PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION );

    $sql = "INSERT INTO medewerkers (voornaam, achternaam, leeftijd, uurloon, opleidingsniveau) VALUES(?, ?, ?, ?, ?)";
    $stmt = $pdo->prepare($sql);
    if ($stmt) {
        $params = [$_POST['voornaam'], $_POST['achternaam'], $_POST['leeftijd'], $_POST['uurloon'], $_POST['opleidingsniveau']];
        $stmt->execute($params);
    }

} catch(Exception $e) {
    echo $e->getMessage();
}

Note that I'm creating a new array for the parameters since I'm not sure if there are more values in the POST. 
