How to create a PHP program for converting Number in Sentence

I want to know the logic of how to creat a program for converting a string number to a sentence, each number represent a alphabet, number are seperated by space. and words in a sentence are seperated by Plus + character.
1=A
2=B ...... 26=Z

and the example input is (16 8 16+3 15 4 5) and the output is "PHP CODE"