Try this:
<?php
if ($_SERVER['REQUEST_METHOD']==='POST') {
$statesArray=$_POST['states'];
print_r($statesArray);
}
?>
<form action="" method="post">
<select name="states[]" multiple>
<option value="AL">Alabama</option>
<option value="AK">Alaska</option>
</select>
<input type="submit">
</form>