How pass data from javascript to php using ajax?

To test if the POST variable has an element called 'userID' you would be better off using array_key_exists .. which actually tests for the existence of the array key not whether its value has been set .. a subtle and probably only semantic difference, but it does improve readability.

and right now your $uid is being set to a boolean value depending whether $__POST['userID'] is set or not ... If I recall from memory you might want to try ...

$uid = (array_key_exists('userID', $_POST)?$_POST['userID']:'guest';

Then you can use an identifiable 'guest' user and render your code that much more readable :)

Another point re isset() even though it is unlikely to apply in this scenario, it's worth remembering if you don't want to get caught out later ... an array element can be legitimately set to NULL ... i.e. it can exist, but be as yet unpopulated, and this could be a valid, acceptable, and testable condition. but :

a = array('one'=>1, 'two'=>null, 'three'=>3);
isset(a['one']) == true
isset(a['two']) == false

array_key_exists(a['one']) == true
array_key_exists(a['two']) == true

Bw sure you know which function you want to use for which purpose.

Today, we’re going to explore the concept of AJAX with PHP and JavaScript. The AJAX  technique helps you to improve your application's user interface and enhance the overall end user experience.

What Is AJAX?

AJAX stands for Asynchronous JavaScript and XML, and it allows you to fetch content from the back-end server asynchronously, without a page refresh. Thus, it lets you update the content of a web page without reloading it.

Let’s look at an example to understand how you could use AJAX in your day-to-day application development. Say you want to build a page that displays a user's profile information, with different sections like personal information, social information, notifications, messages, and so on.

The usual approach would be to build different web pages for each section. So for example, users would click the social information link to reload the browser and display a page with the social information. This makes it slower to navigate between sections, though, since the user has to wait for the browser to reload and the page to render again each time.

On the other hand, you could also use AJAX to build an interface that loads all the information without refreshing the page. In this case, you can display different tabs for all sections, and by clicking on the tab it fetches the corresponding content from the back-end server and updates the page without refreshing the browser. This helps you to improve the overall end-user experience.

The overall AJAX call works something like this:

Let’s quickly go through the usual AJAX flow:

1. First, the user opens a web page as usual with a synchronous request.
2. Next, the user clicks on a DOM element—usually a button or link—that initiates an asynchronous request to the back-end server. The end user won’t notice this since the call is made asynchronously and doesn’t refresh the browser. However, you can spot these AJAX calls using a tool like Firebug.
3. In response to the AJAX request, the server may return XML, JSON, or HTML string data.
4. The response data is parsed using JavaScript.
5. Finally, the parsed data is updated in the web page's DOM.

So as you can see, the web page is updated with real-time data from the server without the browser reloading.

In the next section, we’ll how to implement AJAX using vanilla JavaScript.

How AJAX Works Using Vanilla JavaScript

In this section, we’ll see how AJAX works in vanilla JavaScript. Of course, there are JavaScript libraries available that make it easier to do AJAX calls, but it’s always interesting to know what’s happening under the hood.

Let’s have a look at the following vanilla JavaScript code, which performs the AJAX call and fetches a response from the server asynchronously.

Let’s go through the above code to understand what’s happening behind the scenes.

1. First, we initialize the XMLHttpRequest object, which is responsible for making AJAX calls.
2. The XMLHttpRequest object has a readyState property, and the value of that property changes during the request lifecycle. It can hold one of four values: OPENED, HEADERS_RECEIVED, LOADING, and DONE.
3. We can set up a listener function for state changes using the onreadystatechange property. And that’s what we’ve done in the above example: we’ve used a function which will be called every time the state property is changed.
4. In that function, we’ve checked if the readyState value equals 4, which means the request is completed and we’ve got a response from the server. Next, we’ve checked if the status code equals 200, which means the request was successful. Finally, we fetch the response which is stored in the responseText property of the XMLHttpRequest object.
5. After setting up the listener, we initiate the request by calling the open method of the XMLHttpRequest object. The readyState property value will be set to 1 after this call.
6. Finally, we’ve called the send method of the XMLHttpRequest object, which actually sends the request to the server. The readyState property value will be set to 2 after this call.
7. When the server responds, it will eventually set the readyState value to 4, and you should see an alert box displaying the response from the server.

So that’s how AJAX works with vanilla JavaScript. The method here, using "callback functions" is the traditional way to code AJAX, but a cleaner and more modern way is with Promises.

In the next section, we'll see how to use the Promise object for AJAX.

How to Use JavaScript Promises for AJAX

Promises in JavaScript provide a better way to manage asynchronous operations and callbacks that are dependent on other callbacks. In JavaScript, Promise is an object which may have one of the three states: pending, resolved, or rejected. Initially, the Promise object is in the pending state, but as the asynchronous operation is completed, it may evaluate to the resolved or rejected state.

Let's quickly revise the previous example with the Promise object.

function AjaxCallWithPromise() {
    return new Promise(function (resolve, reject) {
        const objXMLHttpRequest = new XMLHttpRequest();

        objXMLHttpRequest.onreadystatechange = function () {
            if (objXMLHttpRequest.readyState === 4) {
                if (objXMLHttpRequest.status == 200) {
                    resolve(objXMLHttpRequest.responseText);
                } else {
                    reject('Error Code: ' +  objXMLHttpRequest.status + ' Error Message: ' + objXMLHttpRequest.statusText);
                }
            }
        }

        objXMLHttpRequest.open('GET', 'request_ajax_data.php');
        objXMLHttpRequest.send();
    });
}

AjaxCallWithPromise().then(
    data => { console.log('Success Response: ' + data) },
    error => { console.log(error) }
);

When the AjaxCallWithPromise function is called, it returns the promise object, and it's in the pending state initially. Based on the response, it'll call either the resolve or reject function. 

Next, we use the then method, which is used to schedule callbacks when the promise object is successfully resolved. The then method takes two arguments. The first argument is a callback which will be executed when the promise is resolved, and the second argument is a callback for the rejected state.

So that's how you can use JavaScript Promises for AJAX. In the next section, we’ll see how to use the jQuery library to perform AJAX calls.

How AJAX Works Using the jQuery Library

In the earlier section, we discussed how you could perform AJAX calls using vanilla JavaScript. In this section, we’ll use the jQuery library to demonstrate this. I'll assume that you’re aware of the basics of the jQuery library.

The jQuery library provides a few different methods to perform AJAX calls, although here we’ll look at the standard ajax method, which is the most often used.

Take a look at the following example.

As you already know, the $ sign is used to refer to a jQuery object.

The first parameter of the ajax method is the URL that will be called in the background to fetch content from the server side. The second parameter is in JSON format and lets you specify values for some different options supported by the ajax method.

In most cases, you will need to specify the success and error callbacks. The success callback will be called after the successful completion of the AJAX call. The response returned by the server will be passed along to the success callback. On the other hand, the failure callback will be called if something goes wrong and there was an issue performing the AJAX call.

So as you can see, it's easy to perform AJAX operations using the jQuery library. In fact, the process is more or less the same, irrespective of the JavaScript library with which you choose to perform AJAX calls.

In the next section, we’ll see a real-world example to understand how this all works with PHP.

A Real-World AJAX Example With PHP

In this section, we’ll build an example that fetches JSON content from a PHP file on the server side using AJAX.

For demonstration purposes, we'll build an example which performs user login using AJAX and jQuery. To start with, let's make the index.php file, as shown in the following snippet, which renders a basic login form.

    

        Username:
        
        Password:
            
        
    

The index.php file is a pretty standard HTML form which contains username and password fields. It also contains a jQuery JavaScript snippet, which follows the outline we saw above.

We've used the submit event of the form element, which will be triggered when a user clicks on the submit button. In that event handler, we've initiated the AJAX call, which submits the form data to the login.php file using the POST method asynchronously. Once we receive a response from the server, we parse it using the parse method of the JSON object. And finally, based on the success or failure, we take the appropriate action.

Let's also see what login.php looks like.

 1));
} else {
    echo json_encode(array('success' => 0));
}

The login.php file contains the logic of authenticating users and returns a JSON response based on the success or failure of login.

Using Promises for AJAX With jQuery

Apart from this, the $.ajax method supports JavaScript Promises as well. It provides different methods like then, done, fail and always that you could use in the context of Promises.

Let's quickly revise the jQuery snippet which we've used in our example to show how to use it with the then method.

...
...
$.ajax({
    type: "POST",
    url: 'login.php',
    data: $(this).serialize()
}).then(
    // resolve/success callback
    function(response)
    {
        var jsonData = JSON.parse(response);

        // user is logged in successfully in the back-end
        // let's redirect
        if (jsonData.success == "1")
        {
            location.href = 'my_profile.php';
        }
        else
        {
            alert('Invalid Credentials!');
        }
    },
    // reject/failure callback
    function()
    {
        alert('There was some error!');
    }
);
...
...

Conclusion

In this tutorial, we discussed the basics of AJAX and how it works with a PHP app. In the first half of the article, we looked at how AJAX works in vanilla JS and in the jQuery library. In the latter half, we built a real-world example which demonstrated how you can use AJAX to fetch server-side PHP content

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Software Engineer, FSPL, India

I'm a software engineer by profession, and I've done my engineering in computer science. It's been around 14 years I've been working in the field of website development and open-source technologies. Primarily, I work on PHP and MySQL-based projects and frameworks. Among them, I've worked on web frameworks like CodeIgnitor, Symfony, and Laravel. Apart from that, I've also had the chance to work on different CMS systems like Joomla, Drupal, and WordPress, and e-commerce systems like Magento, OpenCart, WooCommerce, and Drupal Commerce. I also like to attend community tech conferences, and as a part of that, I attended the 2016 Joomla World Conference held in Bangalore (India) and 2018 DrupalCon which was held in Mumbai (India). Apart from this, I like to travel, explore new places, and listen to music!

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