Ecco il form:

<form action='pagricevente.php' method='post'>
Cognome: <select name='cognome' value='cognome'>

<?php
$link = mysql_connect("localhost","root","") or die (mysql_error());
mysql_select_db("agenda") or die (mysql_error());

$query = "SELECT cognome " .
"FROM cognome " ;
$result = mysql_query($query,$link) or die (mysql_error());
echo '<option>';
while ($records=mysql_fetch_array($result)) {
echo '<option value=" '.$records[cognome].' ">'.$records[cognome].'</option>';
}
?>
</select>