Displays custom text after the form is submitted with Admin Page Framework, a wordpress plugin/theme framework.

admin-page-framework-custom-text-after-form-submission.php

<?php
/* 
    Plugin Name:    Admin Page Framework - Custom Text After Form Submission
    Plugin URI:     http://en.michaeluno.jp/admin-page-framework
    Description:    Displays custom text after the form is submitted.
    Author:         Michael Uno
*/
 
// Include the library file. Set your file path here.
include( dirname( dirname( __FILE__ ) ) . '/admin-page-framework/library/apf/admin-page-framework.php' );

class APF_CustomTextAfterFormSubmission extends AdminPageFramework {
 
    /**
     * Sets up pages.
     */
    public function setUp() {
       
        $this->setRootMenuPage( 'Settings' ); 
       
        $this->addSubMenuItems(   
            array(
                'title'     => __( 'Custom Text', 'admin-page-framework-demo' ), // page and menu title
                'page_slug' => 'custom_text_after_form_submission',         // page slug
            )
        );   
       
    }
    
    /**
     * Called when the page loads.
     * 
     * @callback        action      load_{page slug}
     */
    public function load_custom_text_after_form_submission( $oAdminPage ) {
        
        $this->addSettingFields(
            array(  
                'field_id'      => 'my_text_field',
                'type'          => 'text',
                'title'         => 'Text',
                'description'   => 'Type something here.', 
            ),
            array( 
                'field_id'      => '_my_custom_field',
                'type'          => '_my_custom_field_for_custom_text',
                'if'            => isset( $_GET[ 'my_custom_key' ] ),
                'content'       => '<p style="color:red; font-weight: bold;">This message is shown after the user submits the form.</p>',
            ),
            array( 
                'field_id'      => 'submit_button',
                'type'          => 'submit',
            )
        );         
        
        add_filter( "setting_update_url_{$oAdminPage->oProp->sClassName}", array( $this, 'replyToModifySettingsUpdateURL' ) );
        
    }
    
    /**
     * @callback    filter      setting_update_url_{class name}
     * @return      string
     */
    public function replyToModifySettingsUpdateURL( $sURL ) {
        return add_query_arg(
            array(
                'my_custom_key' => 'foo',
            ),
            $sURL
        );
    }
   
}
 
new APF_CustomTextAfterFormSubmission;