PHP将<option>数据加载到<textarea>中

<select name="template" id="template">
        <option value="none">Don't use template</option>
        <option value="RWT">Report Abuse</option>
        <option value="renew">Resend Email Codes</option>
    </select><br/>
    <textarea name="body" rows="10" id="content" value="<?php echo $body;?>"ondblclick="select_all('content');" cols="40"></textarea><br/>

....

else if ($template == "renew") {
        $body = "Dear $username,<br/></br/>";
        $body .= "Here is your requested activation code: $activationID <br/><br/>";
        $body .= "Thank you.<br/>";
    }

When I select the option "renew" it doesn't load the data into my textarea. How can I load option data inside the textarea?

It has to go inside the textarea tag:

<textarea name="body" rows="10" id="content" ondblclick="select_all('content');" cols="40">
  <?php echo $body;?>
</textarea>

Update: as tacone mention in the comments, echo htmlspecialchars($body) would be better because it's the right way to do things -- in this specific case there will be no difference, but other times unless you do it your application will be vulnerable to XSS attacks.

Try echoing the template and body variables back to your page for debugging. What are their values?

You don't set the value of a textarea - you simply insert the required content within a pair of textarea elements. For example:

<textarea name="body" id="content" ondblclick="select_all('content');" rows="10" cols="40">
<?php echo $body; ?>
</textarea>