PHP Challenges: Add the digits of a positive integer repeatedly until the result has a single digit

Last update on June 03 2025 12:59:35 (UTC/GMT +8 hours)

Write a PHP program to add the digits of a positive integer repeatedly until the result has a single digit.

Input : 48

For example given number is 59, the result will be 5.
Step 1: 5 + 9 = 14
Step 2: 1 + 4 = 5

Explanation :

PHP: Add the digits of a positive integer repeatedly until the result has single digit

Sample Solution :

PHP Code :

<?php
// Function to add digits of a number until a single digit is obtained
function add_digits($num)
{
    // Check if the number is positive
    if ($num > 0)
    {
        // If positive, return the sum of digits modulo 9 plus 1
        return ($num - 1) % 9 + 1;
    }
    else
    {
        // If negative or zero, return 0
        return 0;
    }
}

// Test the add_digits function with different inputs and print the results
print_r(add_digits(48) . "\n");
print_r(add_digits(59) . "\n");
?>

Explanation:

Here is a brief explanation of the above PHP code:

Function definition:

The code defines a function named "add_digits()" which calculates the digital root of a number. The digital root is obtained by repeatedly adding the digits of the number until a single-digit number is obtained.

Inside the function:

It checks if the input number '$num' is greater than 0.
If the number is positive, it returns the result of the formula ($num - 1) % 9 + 1. This formula calculates the digital root by adding the digits of the number and performing modulo 9 arithmetic.
If the number is non-positive (i.e., negative or zero), it returns 0.

Function Call & Testing:

The "add_digits()" function is called twice with different input numbers (48 and 59).
he results of the function calls are printed using "print_r()", which displays the digital roots of the input numbers.

Sample Output:

3                                                           
5

Flowchart:

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