public function __construct() {
    if (is_admin()) {
        load_plugin_textdomain('Dyslexic_Fonts', false, dirname(plugin_basename(__FILE__)).'/languages');
        add_action('show_user_profile', array($this, 'add_short_form_table_cb'), 5);
        add_action('edit_user_profile', array($this, 'add_short_form_table_cb'), 5); 
        add_action('personal_options_update', array($this, 'save_profile_fields'));
        add_action('edit_user_profile_update', array($this, 'save_profile_fields'));
        add_action('admin_footer', array($this, 'add_font_face'));
    
    } else {
        add_action('wp_footer', array($this, 'add_font_face'));
    }
}

public function add_font_face() {
    global $current_user;
    if ($current_user->ID AND get_user_meta($current_user->ID, '_more_readable', true)) {
        // echo the inline css to load the OpenDyslexic font and apply it via *:not([class*="icon"])
    }
}

public function add_short_form_table_cb() {
    global $current_user; 

    $is_set = get_user_meta($current_user->ID, '_more_readable', true);

    $no = ' selected'; $yes = '';
    if ($is_set) {
        $no = ''; $yes = ' selected';
    }
    
    wp_nonce_field('save_personalisation_meta', '_personalisation');

    $output = '<div id="personalisation">'
            . '<h3>'.__('Site Personalisation', 'Dyslexic_Fonts').'</h3>'
            . '<table class="form-table"><tbody>'
            . '<tr title="'.__('If you have reading difficulties, enable this to use a different font
            	\ across the site.', 'Dyslexic_Fonts').'">'
            . '<th><label for="dyslexic-font">'.__('More readable font?', 'Dyslexic_Fonts').'</label></th>'
            . '<td><select id="dyslexic-font" name="_more_readable">'
            . '<option value="0"'.$no.'>'.__('No', 'Dyslexic_Fonts').'</option><option value="1"'.$yes.'>'
            	\ .__('Yes', 'Dyslexic_Fonts').'</option>'
            . '</select>'
            . '<span class="description">'
            . sprintf(__('This makes the site use a <a href="%s" target="_blank">font designed for people
            	\ with reading difficulties</a>.', 'Dyslexic_Fonts'), 'https://opendyslexic.org/')
            . '</span></td></tr></tbody></table></div>';

    // in case you've got some custom user layout already, here's a filter hook
    $output = apply_filters('dyslexic_fonts', $output, $is_set);

    echo $output;
}

public function save_profile_fields($user_id) {
    if (! current_user_can('edit_user', $user_id) OR ! isset($_POST['_personalisation']) OR 
    	\ ! wp_verify_nonce($_POST['_personalisation'], 'save_personalisation_meta')) {
    	return $user_id;
    }
    
    $set_me = (bool) $_POST['_more_readable'];
    update_user_meta($user_id, '_more_readable', $set_me);
}