Add a trial subscription product to the cart during order processing.

class-nano-trial-subscription.php

<?php
/**
 * When a trial product is purchased we add a subscription product the order
 * directly before order processing.
 *
 * This subscription order charges after 14 days if customer does not cancel.
 */

class Nano_Trial_Subscription {

	/**
	 * The ID for our WooCommerce trial product.
	 * (Trial Kit)
	 *
	 * @since 2.2.0
	 * @access public
	 * @var int
	 */
	public $trial_product_id = 9344;

	/**
	 * The ID for the subscription product.
	 * This product has a trial set, so it won't charge or ship until 14 days out.
	 *
	 * @since 2.2.0
	 * @access public
	 * @var int
	 */
	public $subscription_product_id = 9507;

	/**
	 * The URL for the subscription product.
	 * (Home Bundle)
	 *
	 * @since 2.2.0
	 * @access public
	 * @var string
	 */
	public $subscription_product_url = '/product/for-the-home-bundle/';

	/**
	 * Init the class.
	 *
	 * @since 2.2.0
	 */
	public function init() {

		// The trial product requires additional cart and checkout rules.
		add_filter( 'woocommerce_add_to_cart_validation', array( $this, 'trial_product_cart_filtering' ), 100, 2 );

		// Filters the add to cart message if the trial product is added.
		add_filter( 'wc_add_to_cart_message_html', array( $this, 'trial_add_to_cart_message_html' ), 100, 2 );

		// Adds a field key for trial_agree_to_subscription so it is available in $data for later hooks.
		add_filter( 'woocommerce_checkout_fields' , array( $this, 'trial_agree_to_subscription_field_key' ) );

		// Adds an "agree to subscription" checkbox to the checkout page.
		add_action( 'woocommerce_review_order_before_submit', array( $this, 'trial_agree_to_subscription_checkbox' ) );

		// Saves the value of the "agree to subscription" checkbox to order meta.
		add_action( 'woocommerce_checkout_update_order_meta', array( $this, 'save_agree_to_subscription_meta' ), 100, 2 );

		// Adds the subscription if customer agreed to trial_subscription_accept.
		// This meta field is only on orders that include the trial product.
		add_action( 'woocommerce_checkout_create_order', array( $this, 'check_trial_subscription_accept' ), 100, 2 );

	}

	/**
	 * Helper function to check if cart contains a specific $product_id.
	 *
	 * @since 2.2.0
	 * @param int $product_id
	 * @return boolean
	 */
	function nano_cart_contains( $product_id ) {

		$products = WC()->cart->get_cart_contents();
		foreach ( $products as $product => $values ) {
			$cart_product_id = $values['data']->get_id();
			if ( $product_id === $cart_product_id ) {
				return true;
			}
		}

		return false;

	}

	/**
	 * Restricts which items can be added to cart:
	 *
	 * 1) The $trial_product should not be added to cart if existing items are in the cart.
	 * 2) Other products should not be added to cart if $trial_product is already in cart.
	 *
	 * @since 2.2.0
	 * @param boolean $passed_validation
	 * @param int $product_id
	 * @return boolean
	 */
	function trial_product_cart_filtering( $passed_validation, $product_id ) {

		// If cart is empty, no need for additional validation.
		if ( WC()->cart->is_empty() ) {
			return $passed_validation;
		}

		// If trial product is being added to cart, existing products should be removed.
		// In product details, make sure "Enable this to only allow one of this item to be bought in a single order" is also checked.
		if ( $this->trial_product_id === $product_id ) {
			$notice = sprintf(
				// Translators: Placeholder is for cart url.
				__( 'Trial Product can not be purchased with other products. Please empty your <a href="%s">existing cart</a>.' ),
				esc_url( wc_get_cart_url() )
			);
			wc_add_notice( $notice, 'notice' );
			return false;
		}

		// If trial product is in cart, additional products should not be added.
		if ( $this->nano_cart_contains( $this->trial_product_id ) ) {
			$notice = sprintf(
				// Translators: Placeholder is for cart url.
				__( 'The Trial Product can not be ordered with other items.<br> Please remove the Trial Product from <a href="%s">your cart</a> if you wish to make other purchases.' ),
				esc_url( wc_get_cart_url() )
			);
			wc_add_notice( $notice, 'notice' );
			return false;
		}

		// Trial product is not in cart or being added.
		return $passed_validation;

	}

	/**
	 * Strips out the "Shop" link from add_to_cart message if trial product is added.
	 *
	 * @since 2.2.0
	 * @param string $message
	 * @param array $products
	 * @return string
	 */
	public function trial_add_to_cart_message_html( $message, $products ) {

		// ID of the trial product
		$trial_product_id = 9344;

		// Check cart for trial product
		foreach ( $products as $product_id => $qty ) {
			if ( $this->trial_product_id === $product_id ) {

				// Strips out the "Shop" link/button from the message.
				$message  = preg_replace( '/<a href=\"(.*?)\">(.*?)<\/a>/', '', $message );

				// Adds an additional notice about the subscription product
				$notice = sprintf(
					__( 'After 14 days this order will convert into a subscription for the <a href="%s">Home Bundle</a>. You will be reminded before renewal and can cancel at any time.' ),
					esc_url( home_url( $this->subscription_product_url ) )
				);

				return $message . '<br><br>' . $notice;
			}
		}

		// No changes needed, return default $message.
		return $message;

	}

	public function trial_agree_to_subscription_field_key( $fields ) {
		$fields['order']['trial_subscription_accept']['type'] = 'hidden';
		return $fields;
	}

	/**
	 * Adds an "agree to subscription" checkbox on checkout.
	 *
	 * @since 2.2.0
	 * @return void
	 */
	public function trial_agree_to_subscription_checkbox() {

		if ( $this->nano_cart_contains( $this->trial_product_id ) ) : ?>
			<p class="form-row woocommerce-validated">
				<?php /* Hidden input ensures a value will be posted even if visible input is unchecked. */ ?>
				<input type="hidden" name="trial_subscription_accept" value="0">
				<label>
					<input id="trial-subscription-accept" name="trial_subscription_accept" type="checkbox" value="1" checked>
					<span>I'd like this order to convert into a <a href="<?php echo esc_url( home_url( $this->subscription_product_url ) ); ?> ">Home Bundle</a> subscription after 14 days unless I cancel.</span>
				</label>
			</p>
		<?php endif;

	}

	/**
	 * Saves the meta value of "agree to subscription" checkbox on checkout.
	 *
	 * @since 2.2.0
	 * @return void
	 */
	public function save_agree_to_subscription_meta( $order_id, $data ) {

		$order = wc_get_order( $order_id );

		// The "trial-subscription-accept variable" should only exist on orders with the trial product.
		if ( isset( $_POST['trial_subscription_accept'] ) ) :
			if ( intval( $_POST['trial_subscription_accept'] ) ) {
				$order->update_meta_data( 'trial_subscription_accept', 1 );
			} else {
				$order->update_meta_data( 'trial_subscription_accept', 0 );
			}
			$order->save();
		endif;

	}

	/**
	 * Runs after a payment completes (i.e. complete checkout + payment processed).
	 * If the order contains the trial product, we'll create the subscription order.
	 *
	 * @since 2.2.0
	 * @param int $order_id
	 * @return void
	 */
	public function check_trial_subscription_accept( $order, $data ) {

		// Meta data for subscription approval should only be on orders that contain the trial product.
		$trial_subscription_accept = $order->get_meta( 'trial_subscription_accept' );

		if ( 1 === intval( $data['trial_subscription_accept'] ) ) {
			error_log( 'Scheduling subscription order.' );
			$this->create_subscription_order( $order );
		}

	}

	/**
	 * Create a subscription order with data from the completed trial product order.
	 *
	 * @since 2.2.0
	 * @param object $order
	 * @return void
	 */
	public function create_subscription_order( $order ) {

		// Adds the subscription product to completed order.
		$product = wc_get_product( $this->subscription_product_id );
		$quantity = 1;
		$line_items = array(
			'subtotal' => '0.00',
			'total' => '0.00'
		);

		$order->add_product( $product, $quantity, $line_items );
		WC()->cart->add_to_cart( $this->subscription_product_id, $quantity, array() );

	}

}

// Let's go!
$nano_trial_subscription = new Nano_Trial_Subscription();
$nano_trial_subscription->init();