<script >
            function myFunction() {
                var a = <?php echo json_encode($idkaart[0]); ?>;
                alert( a );

                document.getElementById("klantid").value =  bla[0][0];
            }
        </script>
        <div id="hoofddiv">
            <table class="keuzetabel">
                <form method="post" action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]);?>">
                    <tr><td class="keuzetd">
                        <input type="button" value="Update PHP variable" onclick="myFunction()">
                        </td></tr>
                        <?php
                            $servername = "xx";
                            $username = "xx";
                            $password = "xx";
                            $dbname = "xx";
                            error_reporting(E_ALL);

                            $conn = new mysqli($servername, $username, $password, $dbname);
                            if ($conn->connect_error) {
                                die("Connection failed: " . $conn->connect_error);
                            }//if
                            $sql = "SELECT * FROM `t_klant` WHERE 1 LIMIT 10";
                            $conn->set_charset("utft8");
                            $result = $conn->query($sql);
                            if( $result->num_rows > 0 ) {
                                $i = 0;
                                while( $row = $result->fetch_assoc() ) {
                                    $idkaart[$i] =
                                        array($row["klant_id"],$row["plaats"],$row["land"],$row["mailing"],$row["korting"],$row["tegoed"],$row["status"],
                                        $row["startdatum"],$row["laatstonline"],$row["kolom"],$row["rij"],$row["fontsize"],$row["overbodenmail"]);
                                    //echo $idkaart[$i];
                                    $i += 1;

                                }
                            }

                            $conn->close();
                            for ($i = 0; $i < 10; $i++) {
                                echo "<tr>";
                                for ($j = 0; $j < 13; $j++) {
                                    echo "<td class=\"keuzetd\">" . $idkaart[$i][$j]. "</td>";
                                }//for
                                echo "</tr>";
                            }//for
                            echo $idkaart[0][0] . "<br>";
                            echo json_encode($idkaart);


                        ?>
                </form>

            </table>

        </div>