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digit_sum

Language/Type: PHP recursion return
Author: Eric Roberts (added by Melissa Hovik on 2018/01/05)

Write a recursive function named digit_sum that accepts an integer as a parameter and returns the sum of its digits. For example, calling digit_sum(1729) should return 1 + 7 + 2 + 9, which is 19. If the number is negative, return the negation of the value. For example, calling digit_sum(-1729) should return -19.

Constraints:

  • Do not declare any global variables.
  • Do not use any loops; you must use recursion.
  • Do not use any auxiliary data structures (e.g. arrays) or strings to solve this problem.
  • You can declare as many primitive variables (e.g. integers) as you like.
  • You are allowed to define other "helper" functions if you like; they are subject to these same constraints.
Function: Write a PHP function as described, not a complete program.

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