Application written in pure pgsql, good idea?

Is it a good idea to write a simple application (consisting of just data
entry interfaces) in pure pgsql?

Basically, we would have each page has a stored function in postgresql that
is called by php+apache (the http get/post values would be passed into
postgrel as an array).

The pgpsql would render HTML and return back to the front end for display.

Sample:
create function render_user_login_page(out v_html varchar)
returns varchar
as
$$
begin
v_html := v_html || '<table><tr><td>User ID:</td><td><input
type="text" /></td></tr></table>';
end;
$$

Would there be any performance issues with doing this?