Collapse the Admin Menu by Default

<?php
// WordPress stores the menu-fold preference per user in a user option.
// Returning 'f' (folded) when no value exists collapses the menu by default.
add_filter( 'get_user_option_mfold', 'scseed_collapse_admin_menu_default' );
if ( ! function_exists( 'scseed_collapse_admin_menu_default' ) ) {
    function scseed_collapse_admin_menu_default( $value ) {
        if ( false === $value ) {
            return 'f'; // 'f' = folded; 'o' = open.
        }
        return $value;
    }
}