<?php
/* Plugin Name: Admin Page Framework - Getting Started */ 

include( dirname( __FILE__ ) . '/library/apf/admin-page-framework.php' );
    
class APF extends AdminPageFramework {

    public function setUp() {
        
        $this->setRootMenuPage( 'Settings' );    // where to belong
        $this->addSubMenuItem(
            array(
                'title'        => 'My First Page',
                'page_slug'    => 'myfirstpage'
            )
        );
            
    }
    
    /**
     * @callback        action      do_{page slug}
     */
    public function do_myfirstpage() {
        ?>
        <h3>Say Something</h3>
        <p>This is my first admin page!</p>
        <?php   
    }
    
}
new APF;

<?php
/* Plugin Name: Admin Page Framework - My First Form */ 

include( dirname( __FILE__ ) . '/library/apf/admin-page-framework.php' );
    
class APF_MyFirstFrom extends AdminPageFramework {

    public function setUp() {
        
        $this->setRootMenuPage( 'My Settings' );    // create a root page 
        $this->addSubMenuItem(
            array(
                'title'        => 'My First Form',
                'page_slug'    => 'my_first_form'
            )
        );
                    
    }
    
    /**
     * The pre-defined callback method that is triggered when the page loads.
     * @callback        action      load_{page slug}
     */     
    public function load_my_first_form( $oAdminPage ) {
    
        $this->addSettingSections(    
            array(
                'section_id'    => 'my_first_text_section',    
                'page_slug'     => 'my_first_form',    
            )
        );
        
        $this->addSettingFields(
            array(    
                'field_id'      => 'text',
                'section_id'    => 'my_first_text_section',
                'title'         => 'Text',
                'type'          => 'text',
                'default'       => 123456,
            ),
            array(    
                'field_id'      => 'submit',
                'type'          => 'submit',
            )
        );
        
    }
    
}
new APF_MyFirstFrom;