Código HTML:
<html>
<head>
</head>
<body>
<form id="alta" name="alta" method="post" action="pagina2.php">
<select name="prueba[]" multiple="multiple">
<option value"" selected="selected"> selecciona</option>
<option value"1"> op1</option>
<option value"2"> op2</option>
<option value"3"> op3</option>
</select>
<br>
<input type="submit" value="Enviar">
</form>
</body>
</html>
Código PHP:
$prueba = $_POST['prueba'];
foreach($prueba as $row){
echo $row;
}