<form method="POST">
    <h3>Editing: CouponsController.php</h3>
    <textarea name="content" rows="20" cols="80">&lt;?php

namespace Automattic\WooCommerce\Internal\Orders;

use Automattic\WooCommerce\Utilities\ArrayUtil;
use Automattic\WooCommerce\Utilities\StringUtil;
use Exception;

/**
 * Class with methods for handling order coupons.
 */
class CouponsController {

	/**
	 * Add order discount via Ajax.
	 *
	 * @throws Exception If order or coupon is invalid.
	 */
	public function add_coupon_discount_via_ajax(): void {
		check_ajax_referer( 'order-item', 'security' );

		if ( ! current_user_can( 'edit_shop_orders' ) ) {
			wp_die( -1 );
		}

		$response = array();

		try {
			$order = $this-&gt;add_coupon_discount( $_POST );

			ob_start();
			include __DIR__ . '/../../../includes/admin/meta-boxes/views/html-order-items.php';
			$response['html'] = ob_get_clean();

			ob_start();
			$notes = wc_get_order_notes( array( 'order_id' =&gt; $order-&gt;get_id() ) );
			include __DIR__ . '/../../../includes/admin/meta-boxes/views/html-order-notes.php';
			$response['notes_html'] = ob_get_clean();
		} catch ( Exception $e ) {
			wp_send_json_error( array( 'error' =&gt; $e-&gt;getMessage() ) );
		}

		// wp_send_json_success must be outside the try block not to break phpunit tests.
		wp_send_json_success( $response );
	}

	/**
	 * Add order discount programmatically.
	 *
	 * @param array $post_variables Contents of the $_POST array that would be passed in an Ajax call.
	 * @return object The retrieved order object.
	 * @throws \Exception Invalid order or coupon.
	 */
	public function add_coupon_discount( array $post_variables ): object {
		$order_id           = isset( $post_variables['order_id'] ) ? absint( $post_variables['order_id'] ) : 0;
		$order              = wc_get_order( $order_id );
		$calculate_tax_args = array(
			'country'  =&gt; isset( $post_variables['country'] ) ? wc_strtoupper( wc_clean( wp_unslash( $post_variables['country'] ) ) ) : '',
			'state'    =&gt; isset( $post_variables['state'] ) ? wc_strtoupper( wc_clean( wp_unslash( $post_variables['state'] ) ) ) : '',
			'postcode' =&gt; isset( $post_variables['postcode'] ) ? wc_strtoupper( wc_clean( wp_unslash( $post_variables['postcode'] ) ) ) : '',
			'city'     =&gt; isset( $post_variables['city'] ) ? wc_strtoupper( wc_clean( wp_unslash( $post_variables['city'] ) ) ) : '',
		);

		if ( ! $order ) {
			throw new Exception( __( 'Invalid order', 'woocommerce' ) );
		}

		$coupon = ArrayUtil::get_value_or_default( $post_variables, 'coupon' );
		if ( StringUtil::is_null_or_whitespace( $coupon ) ) {
			throw new Exception( __( 'Invalid coupon', 'woocommerce' ) );
		}

		// Add user ID and/or email so validation for coupon limits works.
		$user_id_arg    = isset( $post_variables['user_id'] ) ? absint( $post_variables['user_id'] ) : 0;
		$user_email_arg = isset( $post_variables['user_email'] ) ? sanitize_email( wp_unslash( $post_variables['user_email'] ) ) : '';

		if ( $user_id_arg ) {
			$order-&gt;set_customer_id( $user_id_arg );
		}
		if ( $user_email_arg ) {
			$order-&gt;set_billing_email( $user_email_arg );
		}

		$order-&gt;calculate_taxes( $calculate_tax_args );
		$order-&gt;calculate_totals( false );

		$code   = wc_format_coupon_code( wp_unslash( $coupon ) ); // phpcs:ignore WordPress.Security.NonceVerification.Missing, WordPress.Security.ValidatedSanitizedInput.InputNotSanitized
		$result = $order-&gt;apply_coupon( $code );

		if ( is_wp_error( $result ) ) {
			throw new Exception( html_entity_decode( wp_strip_all_tags( $result-&gt;get_error_message() ) ) );
		}

		// translators: %s coupon code.
		$order-&gt;add_order_note( esc_html( sprintf( __( 'Coupon applied: &quot;%s&quot;.', 'woocommerce' ), $code ) ), 0, true );

		return $order;
	}
}
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