AJAX in WordPress

wp-ajax-localize-ajaxurl.php

<?php

function example_ajax_enqueue() {
	// Enqueue javascript on the frontend.
	wp_enqueue_script(
		'example-ajax-script',
		get_template_directory_uri() . '/js/simple-ajax-example.js',
		array('jquery')
	);
	// The wp_localize_script allows us to output the ajax_url path for our script to use.
	wp_localize_script(
		'example-ajax-script',
		'example_ajax_obj',
		array( 'ajaxurl' => admin_url( 'admin-ajax.php' ) )
	);
}
add_action( 'wp_enqueue_scripts', 'example_ajax_enqueue' );

wp-ajax-on-frontend.php

<?php

// If you wanted to also use the function for non-logged in users (in a theme for example)
add_action( 'wp_ajax_nopriv_example_ajax_request', 'example_ajax_request' );

wp-ajax-scripts.js

jQuery(document).ready(function($) {
 
    // We'll pass this variable to the PHP function example_ajax_request
    var fruit = 'Banana';
     
    // This does the ajax request
    $.ajax({
        url: ajaxurl, // or example_ajax_obj.ajaxurl if using on frontend
        data: {
            'action': 'example_ajax_request',
            'fruit' : fruit
        },
        success:function(data) {
            // This outputs the result of the ajax request
            console.log(data);
        },
        error: function(errorThrown){
            console.log(errorThrown);
        }
    });  
              
});

wp-ajax-server-side.php

<?php

function example_ajax_request() {
 
    // The $_REQUEST contains all the data sent via ajax
    if ( isset($_REQUEST) ) {
     
        $fruit = $_REQUEST['fruit'];
         
        // Let's take the data that was sent and do something with it
        if ( $fruit == 'Banana' ) {
            $fruit = 'Apple';
        }
     
        // Now we'll return it to the javascript function
        // Anything outputted will be returned in the response
        echo $fruit;
         
        // If you're debugging, it might be useful to see what was sent in the $_REQUEST
        // print_r($_REQUEST);
     
    }
     
    // Always die in functions echoing ajax content
   die();
}
 
add_action( 'wp_ajax_example_ajax_request', 'example_ajax_request' );
 
// If you wanted to also use the function for non-logged in users (in a theme for example)
add_action( 'wp_ajax_nopriv_example_ajax_request', 'example_ajax_request' );