<?php acf_form_head(); ?>
<?php /* Template name: Test */ get_header(); ?>
<?php
$servername = "servername";
    $username = "user";
    $password = "pass";
    $dbname = "name";

    // Create connection
    $conn = mysqli_connect($servername, $username, $password, $dbname);

    // Check connection
    if (!$conn) {
        die("Connection failed... " . mysqli_connect_error());
    }

$FirstName = $_POST["fields"]['acf[field_5968834df2ac1]'];

if(isset($_POST['submit']))
        {

            $sql = "INSERT INTO DBNAME (Roepnaam) VALUES ('$FirstName')";
               if ($conn->query($sql) === TRUE) {
                    echo "";
                } else {
                    echo "Error: " . $sql . "<br>" . $conn->error;
                }
        }

?>

<div id="primary" class="content-area">
        <div id="content" class="site-content" role="main">

            <?php /* The loop */ ?>
            <?php while ( have_posts() ) : the_post(); ?>

                <?php acf_form(array(
                    'post_id'       => 'new_post',
                    'new_post'      => array(
                        'post_type'     => 'db_test',
                        'post_status'       => 'publish'
                    ),
                    'submit_value'      => 'Create a new event'
                )); ?>

            <?php endwhile; ?>

        </div><!-- #content -->
    </div><!-- #primary -->
<?php get_footer(); ?>

<form id="acf-form" class="acf-form" action="" method="post">
    <div class="acf-fields acf-form-fields -top">
        <div id="Firstname" class="acf-field acf-field-text acf-field-5968834df2ac1 voornaam" style="width:40%;" data-name="voornaam" data-type="text" data-key="field_5968834df2ac1" data-width="40">
            <div class="acf-label">
                <label for="acf-field_5968834df2ac1">Voornaam</label>
            </div>
            <div class="acf-input">
                <div class="acf-input-wrap"><input type="text" id="acf-field_5968834df2ac1" class="" name="acf[field_5968834df2ac1]" value="" placeholder="" /></div>   </div>
            </div>
        </div>
    <div class="acf-form-submit">
        <input type="submit" class="acf-button button button-primary button-large" value="Create a new event" />            <span class="acf-spinner"></span>
    </div>
</form>