I have a send button at the end of the form but I do not know how to disable it in the onClick.
echo "<input type='submit' name='add' value=\"".$LANG['help'][14]."\" class='submit' >";
you can use
echo "<input type='submit' name='add' onclick="this.form.submit(); this.disabled = true;" value=\"".$LANG['help'][14]."\" class='submit' >";
You can try following code
echo "<input type='submit' name='add' onclick="this.disabled = true; this.form.submit();" value=\"".$LANG['help'][14]."\" class='submit' >";
please try this
$('input[type="submit"]').click(function() {
this.disabled = true;
};