Registering a New Setting in WordPress

class-sample-plugin.php

<?php
if ( ! class_exists( 'Sample_Plugin' ) ) {
  class Sample_Plugin {
    var $text_domain = 'sample_plugin';
    var $options_page_slug = 'sample-options-page'; /* Because you will use this in a lot of places, we'll define it here */
    function __construct() {
      add_action( 'admin_menu', array( $this, 'admin_menu' ) );
      add_action( 'admin_init', array( $this, 'admin_init' ) );
    }
    
    function admin_menu() {
      add_options_page( 
        /* $page_title = */__( 'Sample Options Page', $this->text_domain ), /* Will be used as <title> element on page, only */
        /* $menu_title = */__( 'Sample Options', $this->text_domain ), /* Will appear as the name within the admin menu */
        /* $capability = */'manage_options', /* Be sure to choose a capability that makes sense for your plugin */
        /* $menu_slug  = */$this->options_page_slug, /* Make this unique, so that no other plugin/theme will clash with it */
        /* $callback   = */array( $this, 'do_options_page' ) /* This will actually output the HTML for the options page */
      );
    }
    
    function admin_init() {
      /* Whitelist the setting itself */
      register_setting( 
        /* $option_group = */$this->options_page_slug, /* For ease of use, use the page slug for this */
        /* $option_name  = */'my-sample-setting', /* Make this unique to your plugin; it will be used as the key in the database */
        /* $callback     = */array( $this, 'sanitize_settings' ) /* This is the function that will be used to escape/sanitize the user input */
      );
      
      /* Add the settings section */
      add_settings_section(
        /* $id           = */'sample-settings-section', /* Make this unique to your plugin */
        /* $title        = */__( 'Sample Settings Section', $this->text_domain ), /* The full text heading for this section */
        /* $callback     = */array( $this, 'settings_section' )
      );
      
      /* Add the settings field */
      add_settings_field(
        /* $id           = */'sample-settings-field', 
        /* $title        = */__( 'Sample Settings Field', $this->text_domain ), 
        /* $callback     = */array( $this, 'settings_field' ), 
        /* $page         = */$this->options_page_slug, /* Make sure this matches the slug you used when regsitering the page */
        /* $section      = */'sample-settings-section', /* Make sure this matches the ID you assigned to the section you regsitered */
        /* $args         = */array( 'label_for' => 'sample-settings-field', 'foo' => 'bar' )
      );
    }
    
    /**
     * Output anything that needs to be at the beginning of our settings section
     * If you want some introductory text or anything like that, use this callback
     *    Otherwise, this can just return null/void.
     */
    function settings_section() {
      return;
    }
    
    /**
     * Output the settings form field itself
     * WordPress handles outputting the label and wrappers, etc.; you just 
     *    need to output the field (and any notes, extra info you need)
     */
    function settings_field( $args=array() ) {
      /* Retrieve the existing values from the database */
      $vals = get_option( 'sample-setting', array() );
      /* You can output any type of field you want here */
      echo '<input type="text" name="settings-field[baz]" id="' . $args['label_for'] . '" value="' . $vals['baz'] . '"/>';
      /* If you need to output any additional info, you can do that, too. You can also access any values sent through the args array */
      echo '<p style="font-style:italic">The value of the foo argument is: ' . $args['foo'] . '</p>';
    }
  }
}