Enable custom fields for the WordPress comments only for pages/posts with a specific custom field key and value. Quick'n'dirty copy + paste out of a plugin I've been working on (also see the debug inserts) ;) In this case, the custom meta field key is $this->pluginPrefix . 'page' = participant_list_page (that should already tell you what I'm aim…

custom-comment-fields-snippet.php

function is_participant_page() {
		global $post;
		$return = false;
		
		$arrDebug = array(
			'post' => $post,
		);
		
		// check custom field
		$strIsParticipantsPage = get_post_meta( $post->ID, $this->pluginPrefix . 'page', true );
		
		$arrDebug['post_meta'] = $strIsParticipantsPage;
		
		if( !empty( $strIsParticipantsPage ) && $strIsParticipantsPage == '1' ) {
			$return = true;
			//echo '<!-- custom post meta: enabled  -->';
		} else {
			//echo '<!-- custom post meta: disabled -->';
		}
		
		//echo '<!-- debug: '. print_r( $arrDebug, true ) . ' -->';
		
		
		return $return;
	}
	
	
	// add a participant to the current list
	
	public function custom_comment_fields( $fields ) {
		$return = $fields;
		
		if( $this->is_participant_page() ) {
			
	
		
			
			$commenter = wp_get_current_commenter();
			$req = get_option( 'require_name_email' );
			$aria_req = ( $req ? " aria-required='true'" : '' );

			$return[ 'author' ] = '<p class="comment-form-author custom-field">'.
				'<label for="author">' . __( 'Name' ) . '</label>'.
				( $req ? '<span class="required">*</span>' : '' ).
				'<input id="author" name="author" type="text" value="'. esc_attr( $commenter['comment_author'] ) . 
				'" size="30" tabindex="1"' . $aria_req . ' /></p>';
			
			$return[ 'email' ] = '<p class="comment-form-email custom-field">'.
				'<label for="email">' . __( 'Email' ) . '</label>'.
				( $req ? '<span class="required">*</span>' : '' ).
				'<input id="email" name="email" type="text" value="'. esc_attr( $commenter['comment_author_email'] ) . 
				'" size="30"  tabindex="2"' . $aria_req . ' /></p>';
						
			$return[ 'url' ] = '<p class="comment-form-url custom-field">'.
				'<label for="url">' . __( 'Website' ) . '</label>'.
				'<input id="url" name="url" type="text" value="'. esc_attr( $commenter['comment_author_url'] ) . 
				'" size="30"  tabindex="3" /></p>';



			$return[ 'phone' ] = '<p class="comment-form-phone custom-field">'.
				'<label for="phone">' . __( 'Phone' ) . '</label>'.
				'<input id="phone" name="phone" type="text" size="30" tabindex="4" /></p>';
		
		}
		
		return $return;
	}