you need to return the form data as an array:
<?php
if (!isset($_POST['submit']))
{
echo '
<form action="" method="POST">
select multiple:<br>
<select name="numbers[]" size="5" multiple>
<option>1
<option>2
<option>3
<option>4
<option>5
</select>
<br>
<input type="submit" name="submit" value="submit">
</form>
';
}
else
{
if (isset($_POST['numbers']))
{
echo 'you selected:<br><br>';
foreach ($_POST['numbers'] as $value) {echo $value . '<br>';}
}
else
{
echo 'you did not select anything';
}
}
?>