<input type="hidden" name="idupdate" value="'. $id. '" /><br>';
Hidden field should be inside your <form>
not after it.
Close the form after your hidden field.
echo '
<font color="white">' . $text. '</font>
<font color="#00FF00">' . $vote . '</font>
<form style="display:inline!important;" id="datplus" name="datplus"
action="mysite.php" method="post" ><input type="submit"
name="submitdatplus" class="submit" value="+" />
<input type="hidden" name="idupdate" value="'. $id. '" /><br>
</form>';