[WordPress] A basic example of how to setup Ajax on both the server-side and the client-side.

00-wp-ajax-server-side.php

<?php

add_action( 'wp_enqueue_scripts', 'acme_add_ajax_library' );
/**
 * Adds the WordPress Ajax Library to the frontend.
 *
 * @since    1.0.0
 */
function acme_add_ajax_library() {

	wp_localize_script(
		'ajax-script',
		'acme_demo',
		array( 'ajax_url' => admin_url( 'admin-ajax.php' ) )
	);

}

/**
 * Determines whether or not the user as voted on the current post. This is used
 * in conjunction with the client-side to prevent users from continually voting on a
 * post.
 * 
 * If the post ID is not set in the $_GET collection, this function will return a -2
 * to indicate that not post ID has been specified.
 *
 * @since    1.0.0
 * @access   private
 *
 * @param    int       $post_id    optional    The ID of the post to determine if the user has voted.
 */
function acme_user_has_voted( $post_id = '' ) {

	// If the post ID isn't specified then retrieve it from the $_GET collection
	if ( '' == $post_id ) {
		
		// Make sure the post ID is set in $_GET
		if ( isset( $_GET['post_id'] ) ) {
			
			/* This might be a little aggressive for this exmaple, but it's shows
			 * why it's important to filter input coming from the client-side.
			 *
			 * ht @gmazzp
			 */
			$post_id = filter_input(
    				INPUT_POST,
    				$_GET['post_id'],
    				FILTER_SANITIZE_NUMBER_INT
			);
		
		} else {
			
			wp_send_json_error(
				new WP_Error( '-2', 'The post ID was not specified.' )
			);
			
		}
		
	}

	// Look to see if there's an IP for this review
	$user_has_voted = ( '' !== trim( get_post_meta( $post_id, "versus_user_has_voted", true ) ) );

	wp_send_json_success(
		json_encode( $user_has_voted )
	);

}

01-acme-set-user-has-voted.js

$.get( ajaxurl, {
  
  action:    'acme_user_has_voted',
  post_id:   $( '#post_id.hidden' ).text()
  
}, function( response ) {

  // Handle the response however best suits your needs
  
});