<?php
 // ------------------------------------------------------------------
 // Add all your sections, fields and settings during admin_init
 // ------------------------------------------------------------------
 //

 function eg_settings_api_init() {
    // Add the section to reading settings so we can add our
    // fields to it
    add_settings_section(
        'eg_setting_section',
        'Example settings section in reading',
        'eg_setting_section_callback_function',
        'reading'
    );

    // Add the field with the names and function to use for our new
    // settings, put it in our new section
    add_settings_field(
        'eg_setting_name',
        'Example setting Name',
        'eg_setting_callback_function',
        'reading',
        'eg_setting_section'
    );

    // Register our setting so that $_POST handling is done for us and
    // our callback function just has to echo the <input>
    register_setting( 'reading', 'eg_setting_name' );
 } // eg_settings_api_init()

 add_action( 'admin_init', 'eg_settings_api_init' );


 // ------------------------------------------------------------------
 // Settings section callback function
 // ------------------------------------------------------------------
 //
 // This function is needed if we added a new section. This function
 // will be run at the start of our section
 //

 function eg_setting_section_callback_function() {
    echo '<p>Intro text for our settings section</p>';
 }

 // ------------------------------------------------------------------
 // Callback function for our example setting
 // ------------------------------------------------------------------
 //
 // creates a checkbox true/false option. Other types are surely possible
 //

 function eg_setting_callback_function() {
    echo '<input name="eg_setting_name" id="eg_setting_name" type="checkbox" value="1" class="code" ' . checked( 1, get_option( 'eg_setting_name' ), false ) . ' /> Explanation text';
 }