Error en variable

1

tengo un error al ejecutar este codigo me sale en la pagina de index este error :
Warning : Use of undefined constant accion - assumed ‘accion’ (this will throw an Error in a future version of PHP) in C:\xampp\htdocs\GestionOdontologica\index.php on line 9

Sistema De Gestion Odontologica <?php 
    if(isset($_GET[accion])){
     if($_GET[accion]="asignar"){
    require_once 'vista/html/asignar.php';  
    } 
    if($_GET[accion]=="consultar"){
    require_once 'vista/html/consultar.php';  
    }
    if($_GET[accion]=="cancelar"){
    require_once 'vista/html/cancelar.php';  
    }
    }
    else{
        require_once 'vista/html/inicio.php';
    }
    ?>
</body>
2

accion is an undefined constant. What you meant to use was 'accion', a string, like:

$_GET['accion']