我想要安装脚本，但出现session_start错误

Question

我遇到此错误 警告：session_start（）：标头已发送

时无法启动会话

这是第84行的代码，我在此出错。

<?php
    session_start();
    function Delete_tables(){
        $conn = mysqli_connect($_SESSION['hostname'], $_SESSION['dbusername'], $_SESSION['dbpassword'], $_SESSION['dbname']);

        $result = mysqli_query($conn, "show tables"); 
        while($table = mysqli_fetch_array($result, MYSQLI_BOTH)) { 
            //echo "<tr><td>".$table[0] . "</td></tr>";             
            $drop_tab = mysqli_query($conn , 'DROP TABLE IF EXISTS `'.$_SESSION['dbname'].'`.`'.$table[0].'`');
        }
        if($drop_tab) {
                //return "<p class='drop-noti'>Tables droped successfully</p><script>jQuery(document).ready(function(){jQuery('.drop-noti').fadeOut(5000);window.location = location.href; });</script>";
                return "<p class='drop-noti'>Tables droped successfully</p><script>jQuery(document).ready(function(){setTimeout(function(){jQuery('.drop-noti').fadeOut(function() {window.location = window.location.href; });}, 5000);});</script>";

            }
            else {
                return mysqli_error($conn); 
            }
    }
    if(isset($_POST['action']) && !empty($_POST['action'])) {
        $action = $_POST['action'];
        switch($action) {
            case 'drop' : $result = Delete_tables();break;
            case 'test' : test();break;
        }
        echo $result; die;
    }       
    ?>