Php 如果用户未从下拉菜单中选择，如何显示回声

在下面的代码中，我有两个下拉菜单。一个是“课程”下拉菜单，另一个是“模块”下拉菜单：

 <?php

         // connect to the database
     include('connect.php');

     /* check connection */
     if (mysqli_connect_errno()) {
    printf("Connect failed: %s\n", mysqli_connect_error());
    die();
    }


    $course = isset($_POST['course']) ? $_POST['course'] : ''; 

    $sql = "
    SELECT cm.CourseId, cm.ModuleId, 
    c.CourseName,
    m.ModuleName
    FROM Course c
    INNER JOIN Course_Module cm ON c.CourseId = cm.CourseId
    JOIN Module m ON cm.ModuleId = m.ModuleId
    WHERE
    (c.CourseId = ?)
    ORDER BY c.CourseId, m.ModuleId
    "; 

     $sqlstmt=$mysqli->prepare($sql);

     $sqlstmt->bind_param("s",$course);

     $sqlstmt->execute(); 

     $sqlstmt->bind_result($dbCourseId,$dbModuleId,$dbCourseName,$dbModuleName);


    $moduleHTML  = "";  

     while($sqlstmt->fetch()) { 
        $moduleHTML .= "<option value='$dbModuleId'>" . $dbModuleId . " - " . $dbModuleName . "</option>".PHP_EOL;   
    } 


    echo $moduleHTML; 

     $sqlstmt->execute(); 


    ?>

代码如下：

   <?php

    // connect to the database
    include('connect.php');

    /* check connection */
    if (mysqli_connect_errno()) {
    printf("Connect failed: %s\n", mysqli_connect_error());
    die();
    }

    $sql = "SELECT CourseId, CourseName FROM Course ORDER BY CourseId"; 

    $sqlstmt=$mysqli->prepare($sql);

    $sqlstmt->execute(); 

    $sqlstmt->bind_result($dbCourseId, $dbCourseName);

    $courses = array(); // easier if you don't use generic names for data 

    $courseHTML = "";  
    $courseHTML .= '<select name="courses" id="coursesDrop" onchange="getModules();">'.PHP_EOL; 
    $courseHTML .= '<option value="">Please Select</option>'.PHP_EOL;  

    while($sqlstmt->fetch()) 
    { 
    $course = $dbCourseId;
    $coursename = $dbCourseName; 
    $courseHTML .= "<option value='".$course."'>" . $course . " - " . $coursename . "</option>".PHP_EOL;  
    } 

    $courseHTML .= '</select>'; 

    $moduleHTML = "";  
    $moduleHTML .= '<select name="modules" id="modulesDrop">'.PHP_EOL; 
    $moduleHTML .= '<option value="">Please Select</option>'.PHP_EOL;  
    $moduleHTML .= '</select>'; 


    include('noscript.php');

    ?>

    <script type="text/javascript">

    function getModules() { 
    var course = jQuery("#coursesDrop").val(); 
    jQuery('#modulesDrop').empty(); 
    jQuery('#modulesDrop').html('<option value="">Please Select</option>'); 
    jQuery.ajax({ 
    type: "post", 
    url:  "module.php", 
    data: { course:course }, 
    success: function(response){ 
    jQuery('#modulesDrop').append(response); 
    } 
    }); 


    }

    </script> 

<h1>EDIT AN ASSESSMENT'S DATE/START TIME</h1>    

<form action="<?php echo htmlentities($_SERVER['PHP_SELF']); ?>" method="post">
<table>
<tr>
<th>Course: <?php echo $courseHTML; ?></th>
<th>Module: <?php echo $moduleHTML; ?></th>
</tr>
</table>
<p><input id="moduleSubmit" type="submit" value="Submit" name="moduleSubmit" /></p>
</form>

<?php

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

$sessionquery = "
SELECT SessionId, SessionDate, SessionTime, ModuleId, TeacherId
FROM Session
WHERE
(ModuleId = ? AND TeacherId = ?)
ORDER BY SessionDate, SessionTime 
";

$sessionqrystmt=$mysqli->prepare($sessionquery);
// You only need to call bind_param once
$sessionqrystmt->bind_param("si",$moduleId,$userid);
// get result and assign variables (prefix with db)

$sessionqrystmt->execute(); 

$sessionqrystmt->bind_result($dbSessionId,$dbSessionDate,$dbSessionTime, $dbModuleId, $dbTeacherId);

$sessionqrystmt->store_result();

$sessionnum = $sessionqrystmt->num_rows();   

if($sessionnum ==0){
echo "<p>Sorry, You have No Sessions under this Module</p>";

以下是HTML代码：

<form action="<?php echo htmlentities($_SERVER['PHP_SELF']); ?>" method="post">
<table>
<tr>
<th>Course: <?php echo $courseHTML; ?></th>
<th>Module: <?php echo $moduleHTML; ?></th>
</tr>
</table>
<p><input id="moduleSubmit" type="submit" value="Submit" name="moduleSubmit" /></p>
<div id="moduleAlert"></div>
</form>

---

您可以使用javascript执行此操作。在javascript中创建一个函数，您可以在其中检查每个字段是否已填充，是否可以像下面这样显示

<script type="text/javascript">
function validate()
{
if(document.dropdownmenuname.course.value=="")
{
alert("Please fill in your form");
document.dropdownmenuname.course.focus() ;
 return false;
}
}
....


函数验证（）
{
if（document.dropdownmenuame.course.value==“”）
{
提醒（“请填写您的表格”）；
document.dropdownmenuame.course.focus（）；
返回false；
}
}
....
更改表单标签

<form action="<?php echo htmlentities($_SERVER['PHP_SELF']); ?>" method="post">

模块ID的来源是什么？大概它应该来自
$\u POST
['modules']`但是我们没有看到这些代码。@MichaelBerkowski模块下拉菜单位于一个单独的页面上，称为module.php，这是通过jquery访问的。这方面的代码我现在已经包含在问题中，你是否在某处调用“validation”函数？@mali303不太清楚，我应该如何以及在哪里调用validation（）函数？我在javascript代码中包含了类似的内容，但不是显示警报，而是显示了一个字符串。问题是我无法显示字符串。我在问题下方添加了一个更新，其中显示了javascript代码和HTML，你能看到我的代码有什么问题吗？为什么不起作用？谢谢，我能问一点吗？我能问一点吗？如果我有单选按钮，我能用php
empty
检查服务器端是否没有选择单选按钮吗？是的，同样，可以在服务器端验证无线电。
<form action="<?php echo htmlentities($_SERVER['PHP_SELF']); ?>" method="post">

<form action="<?php echo htmlentities($_SERVER['PHP_SELF']); ?>" method="post" onsubmit="return validation();">

<?php

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

if(empty($_POST['courses']) && empty($_POST['modules'])) {
   echo 'Please Select a Course and Module';
} elseif(empty($_POST['courses'])) {
   echo 'Please Select a Course';
} elseif(empty($_POST['modules'])) {
   echo 'Please Select a module';
} else {

$sessionquery = "
SELECT SessionId, SessionDate, SessionTime, ModuleId, TeacherId
FROM Session
WHERE
(ModuleId = ? AND TeacherId = ?)
ORDER BY SessionDate, SessionTime 
";

$sessionqrystmt=$mysqli->prepare($sessionquery);
// You only need to call bind_param once
$sessionqrystmt->bind_param("si",$moduleId,$userid);
// get result and assign variables (prefix with db)

$sessionqrystmt->execute(); 

$sessionqrystmt->bind_result($dbSessionId,$dbSessionDate,$dbSessionTime, $dbModuleId, $dbTeacherId);

$sessionqrystmt->store_result();

$sessionnum = $sessionqrystmt->num_rows();   

if($sessionnum ==0){
echo "<p>Sorry, You have No Sessions under this Module</p>";

}