Wordpress Settings API Example Usage

wp-settings-api.php

<?php
/*
Plugin Name: Settings API Example
Plugin URI: http://wpengineer.com/
Description: Complete and practical example of use of the Settings API to add fields on the Writing option page
Author: Ozh
Author URI: http://ozh.org/
*/
// We'll use ozhwpe (Ozh / WP Engineer) as a prefix throughout the plugin
// Register and define the settings
add_action('admin_init', 'ozhwpe_admin_init');
function ozhwpe_admin_init(){
	register_setting(
		'writing',                 // settings page
		'ozhwpe_options',          // option name
		'ozhwpe_validate_options'  // validation callback
	);
	add_settings_field(
		'ozhwpe_notify_boss',      // id
		'Boss Email',              // setting title
		'ozhwpe_setting_input',    // display callback
		'writing',                 // settings page
		'default'                  // settings section
	);
}
// Display and fill the form field
function ozhwpe_setting_input() {
	// get option 'boss_email' value from the database
	$options = get_option( 'ozhwpe_options' );
	$value = $options['boss_email'];
	// echo the field
	?>
<input id='boss_email' name='ozhwpe_options[boss_email]'
 type='text' value='<?php echo esc_attr( $value ); ?>' /> Boss wants to get a mail when a post is published
	<?php
}
// Validate user input
function ozhwpe_validate_options( $input ) {
	$valid = array();
	$valid['boss_email'] = sanitize_email( $input['boss_email'] );
	// Something dirty entered? Warn user.
	if( $valid['boss_email'] != $input['boss_email'] ) {
		add_settings_error(
			'ozhwpe_boss_email',           // setting title
			'ozhwpe_texterror',            // error ID
			'Invalid email, please fix',   // error message
			'error'                        // type of message
		);
	}
	return $valid;
}