To find sum of digits of a number just add all the digits.

For example :

12345= 1 + 2 + 3 + 4 + 5 
12345= 15 

 Logic:

Take the number.
Divide the number by 10.
Add the remainder to a variable.
Repeat the process until remainder is 0.

Example:

Given program shows the sum of digits of 12345.

<?php 
$num = 12345; 
$sum=0; $rem=0; 
  for ($i =0; $i<=strlen($num);$i++) 
 { 
  $rem=$num%10; 
  $sum = $sum + $rem; 
  $num=$num/10; 
  } 
 echo "Sum of digits 12345 is $sum"; 
?> 

Output: Sum of digits 12345 is 15