Allow WordPress user to update category subscriptions

update-subscription.php

<?php
	
	/**
	* Allow users to save categories they wish to subscribe to
	*/
	
	//First make sure the user is logged in
	if( !is_user_logged_in() || !current_user_can('read_posts') ) { //You can change the capability from 'read_posts' if needed
		
		echo '<div class="loginForm">';
			echo '<p>Please login to manage your subscription preferences.</p>';
			wp_login_form();
		echo '</div>';//.loginForm
	}
	
	//The user is logged in
	else {
		$userID = get_current_user_id(); //Get the user's database ID
		
		//Get a list of the terms in the category
		$terms = get_terms('category', array(
			'hide_empty' => false,
			'hierarchical' => false,
		));
		
		//Get the user's current subscription preferences
		$currentCategories = maybe_unserialize(get_user_meta($userID, 'subscription_category', true));
		
		//If no current categories make the var an array so we can use it later
		if( $currentCategories === '' ) $currentCategories = array();
		
		//Show the subscription form
		if( !isset($_POST) || !array_key_exists('subscription_save', $_POST) ) {
			
			echo '<form action="'.get_permalink($post->ID).'" method="post" id="subscriptionSave">';
				echo '<label for="categoryCheckboxes">Choose the categories you wish to subscribe to:</label>';
				
				echo '<div id="categoryCheckboxes">';
				
					//Loop thru each term and provide a checkbox for each
					foreach( $terms as $term ) {
						echo '<input type="checkbox" name="subscriptionCategories['.$term->term_id.']" value="'.$term->term_id.'" '.checked($term->term_id, in_array($term->term_id, $currentCategories), false).' />';
						echo '<label for="subscriptionCategories['.$term->term_id.']">'.$term->name.'</label>';
					}
					
				echo '</div>';//#categoryCheckboxes
				
				//Show a submit button
				echo '<input type="submit" name="subscription_save" value="Save My Preferences" />';
			
			echo '</form>';
			
		}
		
		//Process the subscription form
		else {
			
			//Make sure at least 1 checkbox was selected, if not, delete the user meta
			if( empty($_POST['subscriptionCategories']) ) {
				delete_user_meta($userID, 'subscription_category');
			}
			
			//Get only the term IDs
			$termIDs = get_terms('category', array(
				'hide_empty' => false,
				'hierarchical' => false,
				'fields' => 'ids'
			));
			
			//Check the submission against the list of term IDs to ensure no funny business
			foreach( $_POST['subscriptionCategories'] as $key => $value ) {
				if( !in_array($value, $termIDs) ) unset($_POST['subscriptionCategories'][$key]);
			}
			
			//First we check whether the user already has a meta field saved
			if( empty($currentCategories) ) {
				
				//No user meta exists, use add_user_meta, so we can ensure the field is unique
				if( add_user_meta($userID, 'subscription_category', maybe_serialize($_POST['subscriptionCategories']), true) !== false ) {
					
					//Show a success message
					echo '<div class="message success">';
						echo '<p>Thank you for updating your subscription preferences.</p>';
					echo '</div>';
				}
				
				//Show a failure message
				else {
					echo '<div class="message failure">';
						echo '<p>Something went wrong and we could not save your subscription preferences.</p>';
					echo '</div>';
				}
			}
			
			//The user already has meta saved
			else {
				
				//Update the user meta, because it was created as a unique field, we do not need to specify a previous value.
				if( update_user_meta($userID, 'subscription_category', maybe_serialize($_POST['subscriptionCategories'])) !== false ) {
					
					//Show a success message
					echo '<div class="message success">';
						echo '<p>Thank you for updating your subscription preferences.</p>';
					echo '</div>';
				}
				
				//Show a failure message
				else {
					echo '<div class="message failure">';
						echo '<p>Something went wrong and we could not save your subscription preferences.</p>';
					echo '</div>';
				}
				
			}//if( empty($currentCategories) )
		}//if( !isset($_POST) || !array_key_exists('subscription_save', $_POST) )
	}//if( !is_user_logged_in() || !current_user_can('read_posts') )