Since the introduction of the Fetch API there really is no reason anymore to do this with jQuery Ajax or XMLHttpRequests. To POST form data to a PHP-script in vanilla JavaScript you can do the following:

function postData() {
    const form = document.getElementById('form');
    const data = new FormData();
    data.append('name', form.name.value);

    fetch('../php/contact.php', {method: 'POST', body: data}).then(response => {
        if (!response.ok){
            throw new Error('Network response was not ok.');
        }
    }).catch(err => console.log(err));
}

<form id="form" action="javascript:postData()">
    <input id="name" name="name" placeholder="Name" type="text" required>
    <input type="submit" value="Submit">
</form>

Here is a very basic example of a PHP-script that takes the data and sends an email:

<?php
    header('Content-type: text/html; charset=utf-8');

    if (isset($_POST['name'])) {
        $name = $_POST['name'];
    }

    $to = "[email protected]";
    $subject = "New name submitted";
    $body = "You received the following name: $name";

    mail($to, $subject, $body);

This is a very good article that contains everything that you need to know about jQuery form submission.

Article summary:

Simple HTML Form Submit

HTML:

<form action="path/to/server/script" method="post" id="my_form">
    <label>Name</label>
    <input type="text" name="name" />
    <label>Email</label>
    <input type="email" name="email" />
    <label>Website</label>
    <input type="url" name="website" />
    <input type="submit" name="submit" value="Submit Form" />
    <div id="server-results"><!-- For server results --></div>
</form>

JavaScript:

$("#my_form").submit(function(event){
    event.preventDefault(); // Prevent default action
    var post_url = $(this).attr("action"); // Get the form action URL
    var request_method = $(this).attr("method"); // Get form GET/POST method
    var form_data = $(this).serialize(); // Encode form elements for submission

    $.ajax({
        url : post_url,
        type: request_method,
        data : form_data
    }).done(function(response){ //
        $("#server-results").html(response);
    });
});

HTML Multipart/form-data Form Submit

To upload files to the server, we can use FormData interface available to XMLHttpRequest2, which constructs a FormData object and can be sent to server easily using the jQuery Ajax.

HTML:

<form action="path/to/server/script" method="post" id="my_form">
    <label>Name</label>
    <input type="text" name="name" />
    <label>Email</label>
    <input type="email" name="email" />
    <label>Website</label>
    <input type="url" name="website" />
    <input type="file" name="my_file[]" /> <!-- File Field Added -->
    <input type="submit" name="submit" value="Submit Form" />
    <div id="server-results"><!-- For server results --></div>
</form>

JavaScript:

$("#my_form").submit(function(event){
    event.preventDefault(); // Prevent default action
    var post_url = $(this).attr("action"); // Get form action URL
    var request_method = $(this).attr("method"); // Get form GET/POST method
    var form_data = new FormData(this); // Creates new FormData object
    $.ajax({
        url : post_url,
        type: request_method,
        data : form_data,
        contentType: false,
        cache: false,
        processData: false
    }).done(function(response){ //
        $("#server-results").html(response);
    });
});

I hope this helps.

Please check this. It is the complete Ajax request code.

$('#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();
});

If you want to send data using jQuery Ajax then there is no need of form tag and submit button

Example:

<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" />

Handling Ajax errors and loader before submit and after submitting success shows an alert boot box with an example:

var formData = formData;

$.ajax({
    type: "POST",
    url: url,
    async: false,
    data: formData, // Only input
    processData: false,
    contentType: false,
    xhr: function ()
    {
        $("#load_consulting").show();
        var xhr = new window.XMLHttpRequest();

        // Upload progress
        xhr.upload.addEventListener("progress", function (evt) {
            if (evt.lengthComputable) {
                var percentComplete = (evt.loaded / evt.total) * 100;
                $('#addLoad .progress-bar').css('width', percentComplete + '%');
            }
        }, false);

        // Download progress
        xhr.addEventListener("progress", function (evt) {
            if (evt.lengthComputable) {
                var percentComplete = evt.loaded / evt.total;
            }
        }, false);
        return xhr;
    },
    beforeSend: function (xhr) {
        qyuraLoader.startLoader();
    },
    success: function (response, textStatus, jqXHR) {
        qyuraLoader.stopLoader();
        try {
            $("#load_consulting").hide();

            var data = $.parseJSON(response);
            if (data.status == 0)
            {
                if (data.isAlive)
                {
                    $('#addLoad .progress-bar').css('width', '00%');
                    console.log(data.errors);
                    $.each(data.errors, function (index, value) {
                        if (typeof data.custom == 'undefined') {
                            $('#err_' + index).html(value);
                        }
                        else
                        {
                            $('#err_' + index).addClass('error');

                            if (index == 'TopError')
                            {
                                $('#er_' + index).html(value);
                            }
                            else {
                                $('#er_TopError').append('<p>' + value + '</p>');
                            }
                        }
                    });
                    if (data.errors.TopError) {
                        $('#er_TopError').show();
                        $('#er_TopError').html(data.errors.TopError);
                        setTimeout(function () {
                            $('#er_TopError').hide(5000);
                            $('#er_TopError').html('');
                        }, 5000);
                    }
                }
                else
                {
                    $('#headLogin').html(data.loginMod);
                }
            } else {
                //document.getElementById("setData").reset();
                $('#myModal').modal('hide');
                $('#successTop').show();
                $('#successTop').html(data.msg);
                if (data.msg != '' && data.msg != "undefined") {

                    bootbox.alert({closeButton: false, message: data.msg, callback: function () {
                            if (data.url) {
                                window.location.href = '<?php echo site_url() ?>' + '/' + data.url;
                            } else {
                                location.reload(true);
                            }
                        }});
                } else {
                    bootbox.alert({closeButton: false, message: "Success", callback: function () {
                        if (data.url) {
                            window.location.href = '<?php echo site_url() ?>' + '/' + data.url;
                        } else {
                            location.reload(true);
                        }
                    }});
                }

            }
        }
        catch (e) {
            if (e) {
                $('#er_TopError').show();
                $('#er_TopError').html(e);
                setTimeout(function () {
                    $('#er_TopError').hide(5000);
                    $('#er_TopError').html('');
                }, 5000);
            }
        }
    }
});

I am using this simple one line code for years without a problem (it requires jQuery):

<script type="text/javascript">
    function ap(x,y) {$("#" + y).load(x);};
    function af(x,y) {$("#" + x ).ajaxSubmit({target: '#' + y});return false;};
</script>

Here ap() means an Ajax page and af() means an Ajax form. In a form, simply calling af() function will post the form to the URL and load the response on the desired HTML element.

<form>
    ...
    <input type="button" onclick="af('http://example.com','load_response')"/>
</form>
<div id="load_response">this is where response will be loaded</div>

I use the way shown below. It submits everything like files.

$(document).on("submit", "form", function(event)
{
    event.preventDefault();

    var url  = $(this).attr("action");
    $.ajax({
        url: url,
        type: 'POST',
        dataType: "JSON",
        data: new FormData(this),
        processData: false,
        contentType: false,
        success: function (data, status)
        {

        },
        error: function (xhr, desc, err)
        {
            console.log("error");
        }
    });
});

HTML:

    <form name="foo" action="form.php" method="POST" id="foo">
        <label for="bar">A bar</label>
        <input id="bar" class="inputs" name="bar" type="text" value="" />
        <input type="submit" value="Send" onclick="submitform(); return false;" />
    </form>

JavaScript:

   function submitform()
   {
       var inputs = document.getElementsByClassName("inputs");
       var formdata = new FormData();
       for(var i=0; i<inputs.length; i++)
       {
           formdata.append(inputs[i].name, inputs[i].value);
       }
       var xmlhttp;
       if(window.XMLHttpRequest)
       {
           xmlhttp = new XMLHttpRequest;
       }
       else
       {
           xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
       }
       xmlhttp.onreadystatechange = function()
       {
          if(xmlhttp.readyState == 4 && xmlhttp.status == 200)
          {

          }
       }
       xmlhttp.open("POST", "insert.php");
       xmlhttp.send(formdata);
   }

To make an Ajax request using jQuery you can do this by the following code.

HTML:

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

<!-- The result of the search will be rendered inside this div -->
<div id="result"></div>

JavaScript:

Method 1

 /* Get from elements values */
 var values = $(this).serialize();

 $.ajax({
        url: "test.php",
        type: "post",
        data: values ,
        success: function (response) {

           // You will get response from your PHP page (what you echo or print)
        },
        error: function(jqXHR, textStatus, errorThrown) {
           console.log(textStatus, errorThrown);
        }
    });

Method 2

/* Attach a submit handler to the form */
$("#foo").submit(function(event) {
    var ajaxRequest;

    /* Stop form from submitting normally */
    event.preventDefault();

    /* Clear result div*/
    $("#result").html('');

    /* Get from elements values */
    var values = $(this).serialize();

    /* Send the data using post and put the results in a div. */
    /* I am not aborting the previous request, because it's an
       asynchronous request, meaning once it's sent it's out
       there. But in case you want to abort it you can do it
       by abort(). jQuery Ajax methods return an XMLHttpRequest
       object, so you can just use abort(). */
       ajaxRequest= $.ajax({
            url: "test.php",
            type: "post",
            data: values
        });

    /*  Request can be aborted by ajaxRequest.abort() */

    ajaxRequest.done(function (response, textStatus, jqXHR){

         // Show successfully for submit message
         $("#result").html('Submitted successfully');
    });

    /* On failure of request this function will be called  */
    ajaxRequest.fail(function (){

        // Show error
        $("#result").html('There is error while submit');
    });

The .success(), .error(), and .complete() callbacks are deprecated as of jQuery 1.8. To prepare your code for their eventual removal, use .done(), .fail(), and .always() instead.

MDN: abort() . If the request has been sent already, this method will abort the request.

So we have successfully send an Ajax request, and now it's time to grab data to server.

PHP

As we make a POST request in an Ajax call (type: "post"), we can now grab data using either $_REQUEST or $_POST:

  $bar = $_POST['bar']

You can also see what you get in the POST request by simply either. BTW, make sure that $_POST is set. Otherwise you will get an error.

var_dump($_POST);
// Or
print_r($_POST);

And you are inserting a value into the database. Make sure you are sensitizing or escaping All requests (whether you made a GET or POST) properly before making the query. The best would be using prepared statements.

And if you want to return any data back to the page, you can do it by just echoing that data like below.

// 1. Without JSON
   echo "Hello, this is one"

// 2. By JSON. Then here is where I want to send a value back to the success of the Ajax below
echo json_encode(array('returned_val' => 'yoho'));

And then you can get it like:

 ajaxRequest.done(function (response){
    alert(response);
 });

There are a couple of shorthand methods. You can use the below code. It does the same work.

var ajaxRequest= $.post("test.php", values, function(data) {
  alert(data);
})
  .fail(function() {
    alert("error");
  })
  .always(function() {
    alert("finished");
});

Basic usage of .ajax would look something like this:

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);
    });

});

Note: Since jQuery 1.8, .success(), .error() and .complete() are deprecated in favor of .done(), .fail() and .always().

Note: Remember that the above snippet has to be done after DOM ready, so you should put it inside a $(document).ready() handler (or use the $() shorthand).

Tip: You can chain the callback handlers like this: $.ajax().done().fail().always();

PHP (that is, 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;

Note: Always sanitize posted data, to prevent injections and other malicious code.

You could also use the shorthand .post in place of .ajax in the above JavaScript code:

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

Note: The above JavaScript code is made to work with jQuery 1.8 and later, but it should work with previous versions down to jQuery 1.5.

I would like to share a detailed way of how to post with PHP + Ajax along with errors thrown back on failure.

First of all, create two files, for example form.php and process.php.

We will first create a form which will be then submitted using the jQuery .ajax() method. The rest will be explained in the comments.

form.php

<form method="post" name="postForm">
    <ul>
        <li>
            <label>Name</label>
            <input type="text" name="name" id="name" placeholder="Bruce Wayne">
            <span class="throw_error"></span>
            <span id="success"></span>
       </li>
   </ul>
   <input type="submit" value="Send" />
</form>

Validate the form using jQuery client-side validation and pass the data to process.php.

$(document).ready(function() {
    $('form').submit(function(event) { //Trigger on form submit
        $('#name + .throw_error').empty(); //Clear the messages first
        $('#success').empty();

        //Validate fields if required using jQuery

        var postForm = { //Fetch form data
            'name'     : $('input[name=name]').val() //Store name fields value
        };

        $.ajax({ //Process the form using $.ajax()
            type      : 'POST', //Method type
            url       : 'process.php', //Your form processing file URL
            data      : postForm, //Forms name
            dataType  : 'json',
            success   : function(data) {
                            if (!data.success) { //If fails
                                if (data.errors.name) { //Returned if any error from process.php
                                    $('.throw_error').fadeIn(1000).html(data.errors.name); //Throw relevant error
                                }
                            }
                            else {
                                    $('#success').fadeIn(1000).append('<p>' + data.posted + '</p>'); //If successful, than throw a success message
                                }
                            }
        });
        event.preventDefault(); //Prevent the default submit
    });
});

Now we will take a look at process.php

$errors = array(); //To store errors
$form_data = array(); //Pass back the data to `form.php`

/* Validate the form on the server side */
if (empty($_POST['name'])) { //Name cannot be empty
    $errors['name'] = 'Name cannot be blank';
}

if (!empty($errors)) { //If errors in validation
    $form_data['success'] = false;
    $form_data['errors']  = $errors;
}
else { //If not, process the form, and return true on success
    $form_data['success'] = true;
    $form_data['posted'] = 'Data Was Posted Successfully';
}

//Return the data back to form.php
echo json_encode($form_data);

The project files can be downloaded from http://projects.decodingweb.com/simple_ajax_form.zip.

<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>

You can use serialize. Below is an example.

$("#submit_btn").click(function(){
    $('.error_status').html();
        if($("form#frm_message_board").valid())
        {
            $.ajax({
                type: "POST",
                url: "<?php echo site_url('message_board/add');?>",
                data: $('#frm_message_board').serialize(),
                success: function(msg) {
                    var msg = $.parseJSON(msg);
                    if(msg.success=='yes')
                    {
                        return true;
                    }
                    else
                    {
                        alert('Server error');
                        return false;
                    }
                }
            });
        }
        return false;
    });