變化sqldatabase PHP腳本

Question

我有這個問題，3天已:( 我的phpmyadmin通過創建 3個數據庫和我已經把這些3數據庫到一個下拉列表 我想要的就是當過我點擊提交時，默認數據庫改變，取決於不上的下拉列表中

這個值是我走到

<table width="300" border="0" align="center" cellpadding="0" cellspacing="1"> 
<tr> 
<td><form name="form2" method="post" > 
<table width="100%" border="0" cellspacing="1" cellpadding="3"> 
<tr> 
<td colspan="3"><center><strong>Select Default Database</strong></center></td> 
</tr> 
<tr> 
<td><center><select name="db_used"> 
<?php 
$host="localhost"; 
$username="root"; 
$password=""; 

$link = mysql_connect("$host", "$username", "$password")or die("cannot connect"); 
$result = mysql_list_dbs($link); 

while($row = mysql_fetch_object($result)){ 
echo "<option>$row->Database</option>"; 
} 
?> 

</select></center></td> 
</tr> 
<tr> 
<td colspan="3" align="center"><input type="submit" name="Submit" value="Submit"></td> 
</tr> 
</table> 
</form> 
</td> 
</tr> 
</table> 



<?php 
$host="localhost"; 
$username="root"; 
$password=""; 
$db_name = "#this depends on the value of the dropdown list"; 

mysql_connect("$host", "$username", "$password")or die("cannot connect"); 
mysql_select_db("$db_name")or die("cannot select DB"); 
?> 

代碼任何幫助是極大的讚賞~~