Add tags before and after the highlighted text

Hi, I'm making my own BBcode and I want to make something that will enter tags like when the user highlights a text they want to add the tags too, like in this forums. I have this script but it enters the tags in front of the text..

<form method="post" name="addcomment">
Comment:

<input type="button" onClick="document.addcomment.comment.value+=\'\'" value="Bold" name="bold" title="Add Bold" >

<input type="button" onClick="document.addcomment.comment.value+=\'\'" value="Italic" name="italic" title="Add Italic" >

<input type="button" onClick="document.addcomment.comment.value+=\'[php][/php]\'" value="PHP Code" name="php" title="Add PHP Code" >

<input type="button" onClick="document.addcomment.comment.value+=\'[url=http://linkhere.com]Link text[/url]\'" value="URL" name="url" title="Add Hyperlink" >

<textarea rows="5" cols="35" name="comment"></textarea>

<input type="submit" name="add_comment" value="Submit">
</form>

Code (markup):

Thanks in advance .

 

Call a handler script, don't try to do it all with onclick, you'll just make yourself crazy.