wordpress help... integrating existing PHP code to work side-by-side -

i know little of wp aside form being cms geared towards (or started from) blogging, may people have found product capable of functioning sites cms.

i asked write php app signup, (with email confirm , email notification admin), login make , manage orders. - user can register , email confirmation... once approved, can log in, , place order. , manage information. there admin section manage users , requests... straight ahead.

so write - , test , fine... until client tells me it's going part of wp site.

problem, client knows html, not php... don't know wp.

when upload directory root - , try run app, redirect /$url .. , page not found displaying in wp theme.

i have feeling has auth module i'm using... there huge big picture issue need conquer - how integrate existing php app wp site...

q: how reference , use wp emailing system?

thx - know it's broad question. if can point me direction... have read post regarding templates in wp , setting template php code it's executed... seems 'wrong' have create template each php page.

what app ? if got auth module coded should import user , password because wp out-of-the-box.

wordpress can twist need need in wp way :).

if want add functions check out plugin library on wordpress.org. if know wordpress , no plugin match needs best way go writing own plugin : https://codex.wordpress.org/writing_a_plugin

or maybe add custom functions functions.php, see https://codex.wordpress.org/theme_development#functions_file

for pages, have type of in wp : articles - i.e. blog posts - , static pages. can add custom 1 check https://codex.wordpress.org/post_types

i know answer more bunch of links if don't nothing wp should first learn how works before try hack it.

hope helps !