Accès à une autre page

if(isset($_POST['Valider']))   { 
   //récupération PROPRE des variables AVANT de les utiliser
   $user=!empty($_POST['user']) ? $_POST['user']: NULL;
   $password= !empty($_POST['password']) ? $_POST['password']: NULL;

    // ici ton code pour vérifier les identifiants...
   
    // puis .. si utilisateur est connecté
    if( $userconnecte ){
       //ici la redirection si l'utilisateur est connecté
       header('location : tapage.php');
       exit(); //toujours mettre un exit apres une redirection
    }
}

<?php
/*
Plugin Name:Gestion_salles
Description:Plugin pour la gestion des salles de cours
Version:0.1
Licence:GPL
Author:Alidi
Author URL:http://localhost/wordpress
*/

//don't load directly
if(!defined('ABSPATH'))
  die('-1');
  
  //Les constantes
  define('DEV_URL',plugin_dir_url (__FILE__));
  define('DEV_DIR',plugin_dir_path(__FILE__));
  define('DEV_VERSION', '0.1');
  define('DEV_NAME', 'Gestion_salles' );
  define('DEV_OPTION_SETTINGS', 'Gestion_salles_settings');


// Menu Admin Activation

add_action('admin_menu','add_admin_menu');
function add_admin_menu(){
//parameters details
//add_management_page($page_title, $menu_title, $capability, $menu_slug, $function );
//add a new setting page udner setting menu
add_menu_page('My plugin' ,'Gestion_salles', 'administrator', 'menu_Gestion_salles', 'Gestion_init','dashicons-tickets', 6 );
}
function Gestion_init(){


		function wp_authenticate($username, $password) {
    $username = sanitize_user($username);
    $password = trim($password);
 
    /**
     * Filters whether a set of user login credentials are valid.
     *
     * A WP_User object is returned if the credentials authenticate a user.
     * WP_Error or null otherwise.
     *
     * @since 2.8.0
     * @since 4.5.0 `$username` now accepts an email address.
     *
     * @param null|WP_User|WP_Error $user     WP_User if the user is authenticated.
     *                                        WP_Error or null otherwise.
     * @param string                $username Username or email address.
     * @param string                $password User password
     */
    $user = apply_filters( 'authenticate', null, $username, $password );
 
    if ( $user == null ) {
        // TODO what should the error message be? (Or would these even happen?)
        // Only needed if all authentication handlers fail to return anything.
        $user = new WP_Error( 'authentication_failed', __( '<strong>ERROR</strong>: Invalid username, email address or incorrect password.' ) );
    }
 
    $ignore_codes = array('empty_username', 'empty_password');
 
    if (is_wp_error($user) && !in_array($user->get_error_code(), $ignore_codes) ) {
        /**
         * Fires after a user login has failed.
         *
         * @since 2.5.0
         * @since 4.5.0 The value of `$username` can now be an email address.
         *
         * @param string $username Username or email address.
         */
        do_action( 'wp_login_failed', $username );
    }
 
    return $user;
}
	

}
?>