Wordpress Use the settings API

PHP

<?php
/*
 * Add the admin page
 */
add_action('admin_menu', 'wpse61431_admin_page');
function wpse61431_admin_page(){
    add_menu_page('wpse61431 Settings', 'wpse61431', 'administrator', 'wpse61431-settings', 'wpse61431_admin_page_callback');
}

/*
 * Register the settings
 */
add_action('admin_init', 'wpse61431_register_settings');
function wpse61431_register_settings(){
    //this will save the option in the wp_options table as 'wpse61431_settings'
    //the third parameter is a function that will validate your input values
    register_setting('wpse61431_settings', 'wpse61431_settings', 'wpse61431_settings_validate');
}

function wpse61431_settings_validate($args){
    //$args will contain the values posted in your settings form, you can validate them as no spaces allowed, no special chars allowed or validate emails etc.
    if(!isset($args['wpse61431_email']) || !is_email($args['wpse61431_email'])){
        //add a settings error because the email is invalid and make the form field blank, so that the user can enter again
        $args['wpse61431_email'] = '';
    add_settings_error('wpse61431_settings', 'wpse61431_invalid_email', 'Please enter a valid email!', $type = 'error');   
    }

    //make sure you return the args
    return $args;
}

//Display the validation errors and update messages
/*
 * Admin notices
 */
add_action('admin_notices', 'wpse61431_admin_notices');
function wpse61431_admin_notices(){
   settings_errors();
}

//The markup for your plugin settings page
function wpse61431_admin_page_callback(){ ?>
    <div class="wrap">
    <h2>wpse61431 Settings</h2>
    <form action="options.php" method="post"><?php
        settings_fields( 'wpse61431_settings' );
        do_settings_sections( __FILE__ );

        //get the older values, wont work the first time
        $options = get_option( 'wpse61431_settings' ); ?>
        <table class="form-table">
            <tr>
                <th scope="row">Email</th>
                <td>
                    <fieldset>
                        <label>
                            <input name="wpse61431_settings[wpse61431_email]" type="text" id="wpse61431_email" value="<?php echo (isset($options['wpse61431_email']) && $options['wpse61431_email'] != '') ? $options['wpse61431_email'] : ''; ?>"/>
                            <br />
                            <span class="description">Please enter a valid email.</span>
                        </label>
                    </fieldset>
                </td>
            </tr>
        </table>
        <input type="submit" value="Save" />
    </form>
</div>
<?php }
?>