请核对一下。它是完整的Ajax请求代码。

$('#foo').submit(function(event) {
    // Get the form data
    // There are many ways to get this data using jQuery (you
    // can use the class or id also)
    var formData = $('#foo').serialize();
    var url = 'URL of the request';

    // Process the form.
    $.ajax({
        type        : 'POST',   // Define the type of HTTP verb we want to use
        url         : 'url/',   // The URL where we want to POST
        data        : formData, // Our data object
        dataType    : 'json',   // What type of data do we expect back.
        beforeSend : function() {

            // This will run before sending an Ajax request.
            // Do whatever activity you want, like show loaded.
        },
        success:function(response){
            var obj = eval(response);
            if(obj)
            {
                if(obj.error==0){
                    alert('success');
                }
                else{
                    alert('error');
                }
            }
        },
        complete : function() {
            // This will run after sending an Ajax complete
        },
        error:function (xhr, ajaxOptions, thrownError){
            alert('error occured');
            // If any error occurs in request
        }
    });

    // Stop the form from submitting the normal way
    // and refreshing the page
    event.preventDefault();
});

ajax的基本用法是这样的:

HTML:

<form id="foo">
    <label for="bar">A bar</label>
    <input id="bar" name="bar" type="text" value="" />

    <input type="submit" value="Send" />
</form>

jQuery:

// Variable to hold request
var request;

// Bind to the submit event of our form
$("#foo").submit(function(event){

    // Prevent default posting of form - put here to work in case of errors
    event.preventDefault();

    // Abort any pending request
    if (request) {
        request.abort();
    }
    // setup some local variables
    var $form = $(this);

    // Let's select and cache all the fields
    var $inputs = $form.find("input, select, button, textarea");

    // Serialize the data in the form
    var serializedData = $form.serialize();

    // Let's disable the inputs for the duration of the Ajax request.
    // Note: we disable elements AFTER the form data has been serialized.
    // Disabled form elements will not be serialized.
    $inputs.prop("disabled", true);

    // Fire off the request to /form.php
    request = $.ajax({
        url: "/form.php",
        type: "post",
        data: serializedData
    });

    // Callback handler that will be called on success
    request.done(function (response, textStatus, jqXHR){
        // Log a message to the console
        console.log("Hooray, it worked!");
    });

    // Callback handler that will be called on failure
    request.fail(function (jqXHR, textStatus, errorThrown){
        // Log the error to the console
        console.error(
            "The following error occurred: "+
            textStatus, errorThrown
        );
    });

    // Callback handler that will be called regardless
    // if the request failed or succeeded
    request.always(function () {
        // Reenable the inputs
        $inputs.prop("disabled", false);
    });

});

注意:从jQuery 1.8开始，.success()， .error()和.complete()已被弃用，取而代之的是.done()， .fail()和.always()。

注意:请记住，上面的代码段必须在DOM ready之后完成，因此应该将其放在$(document).ready()处理程序中(或使用$()简写)。

提示:你可以像这样链接回调处理程序:$.ajax().done().fail().always();

PHP(即form.php):

// You can access the values posted by jQuery.ajax
// through the global variable $_POST, like this:
$bar = isset($_POST['bar']) ? $_POST['bar'] : null;

注意:总是消毒发布的数据，以防止注入和其他恶意代码。

你也可以在上面的JavaScript代码中使用。post来代替。ajax:

$.post('/form.php', serializedData, function(response) {
    // Log the response to the console
    console.log("Response: "+response);
});

注意:上面的JavaScript代码适用于jQuery 1.8及更高版本，但它应该适用于jQuery 1.5之前的版本。

如果你想用jQuery Ajax发送数据，那么就不需要表单标签和提交按钮

例子:

<script>
    $(document).ready(function () {
        $("#btnSend").click(function () {
            $.ajax({
                url: 'process.php',
                type: 'POST',
                data: {bar: $("#bar").val()},
                success: function (result) {
                    alert('success');
                }
            });
        });
    });
</script>

<label for="bar">A bar</label>
<input id="bar" name="bar" type="text" value="" />
<input id="btnSend" type="button" value="Send" />

这是使用ajax在HTML中填充选择选项标签的代码，而XMLHttpRequest和API是用PHP和PDO编写的

conn.php

<?php
$servername = "localhost";
$username = "root";
$password = "root";
$database = "db_event";
try {
    $conn = new PDO("mysql:host=$servername;dbname=$database", $username, $password);
    $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
} catch (PDOException $e) {
    echo "Connection failed: " . $e->getMessage();
}
?>

category.php

<?php
 include 'conn.php';
try {
    $data = json_decode(file_get_contents("php://input"));
    $stmt = $conn->prepare("SELECT *  FROM events ");
    http_response_code(200);
    $stmt->execute();
    
    header('Content-Type: application/json');

    $arr=[];
    while($value=$stmt->fetch(PDO::FETCH_ASSOC)){
        array_push($arr,$value);
    }
    echo json_encode($arr);
   
  } catch(PDOException $e) {
    echo "Error: " . $e->getMessage();
  }

script.js

var xhttp = new XMLHttpRequest();
xhttp.onreadystatechange = function () {
    if (this.readyState == 4 && this.status == 200) {
        data = JSON.parse(this.responseText);

        for (let i in data) {


            $("#cars").append(
                '<option value="' + data[i].category + '">' + data[i].category + '</option>'

            )
        }
    }
};
xhttp.open("GET", "http://127.0.0.1:8000/category.php", true);
xhttp.send();

index . html

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <link rel="stylesheet" href="style.css">
    <script src="https://code.jquery.com/jquery-3.6.0.min.js"
        integrity="sha256-/xUj+3OJU5yExlq6GSYGSHk7tPXikynS7ogEvDej/m4=" crossorigin="anonymous"></script>
    <title>Document</title>
</head>

<body>
    <label for="cars">Choose a Category:</label>

    <select name="option" id="option">
        
    </select>
    
    <script src="script.js"></script>
</body>

</html>

我还有一个主意。

提供下载文件的PHP文件的URL。 然后你必须通过ajax触发相同的URL，我检查了这第二个请求只在你的第一个请求完成下载文件后才给出响应。所以你可以得到它的事件。

它正在通过ajax处理同样的第二个请求。

<script src="http://code.jquery.com/jquery-1.7.2.js"></script>
<form method="post" id="form_content" action="Javascript:void(0);">
    <button id="desc" name="desc" value="desc" style="display:none;">desc</button>
    <button id="asc" name="asc"  value="asc">asc</button>
    <input type='hidden' id='check' value=''/>
</form>

<div id="demoajax"></div>

<script>
    numbers = '';
    $('#form_content button').click(function(){
        $('#form_content button').toggle();
        numbers = this.id;
        function_two(numbers);
    });

    function function_two(numbers){
        if (numbers === '')
        {
            $('#check').val("asc");
        }
        else
        {
            $('#check').val(numbers);
        }
        //alert(sort_var);

        $.ajax({
            url: 'test.php',
            type: 'POST',
            data: $('#form_content').serialize(),
            success: function(data){
                $('#demoajax').show();
                $('#demoajax').html(data);
                }
        });

        return false;
    }
    $(document).ready(function_two());
</script>