A word that describes a core feature involved?
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.
Crux might be useful to you:
The basic, central, or essential point or feature.