php vs other modern language

I am confused by something. As some of you know I have been a programmer for many decades. I initially worked on ibm minis before the birth of the PC world. I then was a C programmer for another decade or two. In all of the older languages you could have a third party write some code that we could then call from the main source code and it would run. There were of course other steps involved such as compiling, linking, etc but the bottom line is that there was a "modular" way to build an app. 
Does CI3 have the ability to call an independently written, php functions (not js) in a modular fashion? 

Sorry for the stupid question. I just can't believe that we can no longer do that?

proof that an old dog can learn new tricks