PHP Program to Find Sum of Digits of a Number

You can find the sum of digits in PHP by converting the number to a string and adding each digit using a loop, like $sum = 0; foreach (str_split($num) as $digit) { $sum += (int)$digit; }.

Examples

Input123

Output6

Input405

Output9

Input0

Output0

How to Think About It

To find the sum of digits, think of the number as a collection of single digits. You add each digit one by one to a total sum. You can do this by turning the number into a string and looping through each character, or by repeatedly taking the last digit using % and removing it using / until the number is zero.

Algorithm

1

Get the input number.

2

Initialize a sum variable to zero.

3

While the number is greater than zero:

4

Extract the last digit using modulus 10.

5

Add the digit to the sum.

6

Remove the last digit by dividing the number by 10 and taking the integer part.

7

Return or print the sum.

Code

php

<?php
$num = 123;
$sum = 0;
while ($num > 0) {
    $digit = $num % 10;
    $sum += $digit;
    $num = (int)($num / 10);
}
echo $sum;
?>

Output

6

Dry Run

Let's trace the number 123 through the code to find the sum of its digits.

1

Initial values

$num = 123, $sum = 0

2

First iteration

digit = 123 % 10 = 3; sum = 0 + 3 = 3; num = 123 / 10 = 12

3

Second iteration

digit = 12 % 10 = 2; sum = 3 + 2 = 5; num = 12 / 10 = 1

4

Third iteration

digit = 1 % 10 = 1; sum = 5 + 1 = 6; num = 1 / 10 = 0

5

Loop ends

num is now 0, loop stops; final sum = 6

Iteration	num	digit	sum
1	123	3	3
2	12	2	5
3	1	1	6

Why This Works

Step 1: Extract each digit

Using % 10 gets the last digit of the number because it gives the remainder when divided by 10.

Step 2: Add digit to sum

We add the extracted digit to the running total stored in $sum.

Step 3: Remove last digit

Dividing the number by 10 and taking the integer part removes the last digit, preparing for the next loop iteration.

Alternative Approaches

String conversion and loop

php

<?php
$num = 123;
$sum = 0;
foreach (str_split((string)$num) as $digit) {
    $sum += (int)$digit;
}
echo $sum;
?>

This method is simple and easy to read but uses extra memory to convert the number to a string.

Recursive function

php

<?php
function sumDigits($num) {
    if ($num == 0) return 0;
    return ($num % 10) + sumDigits((int)($num / 10));
}
echo sumDigits(123);
?>

This uses recursion to sum digits but can be less efficient for very large numbers due to call stack overhead.

Complexity: O(n) time, O(1) space

Time Complexity

The loop runs once for each digit in the number, so time grows linearly with the number of digits.

Space Complexity

Only a few variables are used, so space is constant regardless of input size.

Which Approach is Fastest?

The modulus and division loop is fastest and uses least memory; string conversion is simpler but uses extra space; recursion is elegant but less efficient.

Approach	Time	Space	Best For
Modulus and division loop	O(n)	O(1)	Performance and low memory
String conversion and loop	O(n)	O(n)	Readability and simplicity
Recursive function	O(n)	O(n)	Learning recursion and elegance