Replace string letters with nth alphabet character in PHP -

How do I change letters in letters with their + N cospaintent?

For example, replace each character with its +4Cospendent, as shown below:

  abcdefghijklmnopqrstu vwxyz â ???? a???? a???? a???? a???? a???? a???? a???? a???? a???? a???? a???? a???? a???? a???? a???? a???? a???? a???? a???? a???? a???? a???? a???? a???? a???? Efghijklmnopqrstuvwxy zabcd   
 Therefore, if I have the string  johnny , then it should be  nslrrc .   
 
 
 You can create a character-of-character substitution with:  
  $ shiftBy = 4; $ Alphabet = 'abcdefghijklmnopqrstuvwxyz'; $ NewAlpha = substr ($ alphabet, $ shiftBy). Substr ($ alphabet, 0, shift shift); Echo Strat ("Johnny", $ alphabet, $ newAlpha); // nslrrc   
 Of course, it considers all the lowercase examples of your example. Politics complicates things.  
  
  Bonus: also works with negative changes