Q:

A word that describes a core feature involved?

I have made an application which is mostly programmed in PHP, but have also used some other languages. Now I want to describe that I have made a program that is "corely" programmed in PHP, but still remember that my application have mostly made on PHP.
Corely is not in dictionary. What should be a word for that?

A:

You could use predominantly, primarily or principally to say either the largest or most important portion is written in PHP, while quantity alone can be stressed with words like chiefly, mainly, mostly, or largely.
If you want to say the main or most important part of the code is in PHP and peripheral functionality in other languages, you could reword along the lines of 

The core of the code is written in PHP.

