Improve documentation for hidden fields by "Conditional Fields for Co…

…ntact Form 7" plugin.

.wp-env.json

@@ -6,6 +6,7 @@
 		"./tests/wp-env-mail.php",
 		"pronamic/wp-pronamic-pay-test-helper",
 		"https://downloads.wordpress.org/plugin/contact-form-7.5.6.2.zip",
+		"https://downloads.wordpress.org/plugin/cf7-conditional-fields.zip",
 		"https://downloads.wordpress.org/plugin/flamingo.zip",
 		"https://downloads.wordpress.org/plugin/pronamic-client.zip",
 		"https://downloads.wordpress.org/plugin/pronamic-ideal.zip",

README.md

@@ -32,3 +32,4 @@ skip_mail: on
 - https://wordpress.org/plugins/pronamic-pay-with-mollie-for-contact-form-7/
 - https://wordpress.org/plugins/contact-form-7/
 - https://github.com/rocklobster-in/contact-form-7
+- https://wordpress.org/plugins/cf7-conditional-fields/

src/Pronamic.php

@@ -55,7 +55,12 @@ public static function get_submission_value( WPCF7_Submission $submission, $type
 
 		$prefixed_type = 'pronamic_pay_' . $type;
 
-		// Hidden fields.
+		/**
+		 * Hidden fields may arise when using the "Conditional Fields for Contact Form 7" plugin.
+		 * 
+		 * @link https://wordpress.org/plugins/cf7-conditional-fields/
+		 * @link https://github.com/pronamic/wp-pronamic-pay-contact-form-7/commit/83122efa3755f1d4b667aed3e3e7c2ae0f813faa
+		 */
 		$hidden_fields = \filter_input( \INPUT_POST, '_wpcf7cf_hidden_group_fields' );
 
 		if ( ! empty( $hidden_fields ) ) {