Some editors - like JCE - interpret html tags even though you paste them into the WYSIWYG editor. So it wil also strip unwanted code (like scripts). This is very irritating, but anyway.
For that reason, Sourcerer! supports the
double bracket syntax. All you need to do in your code, is (for tags) replace all '<' with '[[' and all '>' with ']]'.
So your code should look like this:
{source}[[?php include($_SERVER['DOCUMENT_ROOT']."/templates/username/includes/
form.php"); ?]]{/source}
It is no problem to use enters within the Sourcerer! tags, because it interprets all like source code. So for readability you can also do this:
{source}
[[?php
include($_SERVER['DOCUMENT_ROOT']."/templates/username/includes/form.php");
?]]
{/source}