如何创建一个填充的下拉列表，该列表将根据他们所做的下拉列表选择自动填充表单 - 从数据库中获取

如何创建一个填充的下拉列表，该列表将根据他们所做的下拉列表选择自动填充表单 - 从数据库中获取

我正在研究这个，对 php 很陌生，但这是我到目前为止的代码

$hostname="";
$username="";
$password="";
$database="";
mysql_connect($hostname,$username,$password);
@mysql_select_db($database) or die( "Unable to select database");
//getFN();
function getFN(){
$query = "SELECT first FROM contacts";
$FNresult=mysql_query($query); 

$dropdown = "<select name='contacts' >";
while($row = mysql_fetch_assoc($FNresult)) {
  $dropdown .= "'r'n<option value='{$row['first']}'>{$row['first']}</option>";
  echo getLN();
  //$last .="'r'n<option value='{$row['last']}'>{$row['last']}</option>";
//echo $last;
}
$dropdown .= "'r'n</select>";
echo $dropdown;
}
//                  Get last name
function getLN(){
    $query = "SELECT last FROM contacts";
    $LNresult=mysql_query($query);
    $last;
    while($row = mysql_fetch_assoc($LNresult)) {
        $last = "{$row['last']}";
    }
    echo $last;
}//end getLN

mysql_close();
?>
<select name="fdsfd" onchange="document.getElementById('first').value = this.value">
<!-- <option value="<?//php echo $first; ?>"></option>-->
</select>

<form action="insert.php" method="post">
First Name: <input type="text" id="first" value=""><br>
Last Name: <input type="text" id="last"><br>
Phone: <input type="text" id="phone"><br>
Mobile: <input type="text" id="mobile"><br>
Fax: <input type="text" id="fax"><br>
E-mail: <input type="text" id="email"><br>
Web: <input type="text" id="web"><br>
<input type="Submit">
</form>