$_SESSION['dataArray] = array('stap' => 'stap1', 'stijlkeuze' => 'foo');

switch($_SESSION['dataArray']['stap']) {
	case 'stap1':

		if ( isset($_POST['submit']) ) {

			$_SESSION['dataArray']['stijlkeuze'] = $_POST['stijlkeuze'];

			$_SESSION['dataArray']['stap'] = 'stap2';

		}

		php

		<form method="post">
			<input type="text" value="<?=$_SESSION['dataArray']['stijlkeuze']?>" name="stijlkeuze" />
			<input type="submit" name="submit" />
		</form>

	break;
	case 'stap2':
	break;
}