recursive_sum.php

<?php
function reduce ($_int, $counter = 0) {
    echo 'number : ' . $_int . ' <br> ' . 'counter: ' . $counter . ' <br> ----------------- <br>';

    $intArray = preg_split('//', (string)$_int, -1, PREG_SPLIT_NO_EMPTY);

    if(count($intArray) > 1){
       return reduce ( array_sum($intArray), ($counter + $_int) );
    } else {
       return $counter + $_int; 
    } 

}

print_r(reduce(9875));

SumRecursive.php

<?php
class SumRecursive
{
    public $num;

    public function __construct($num) 
    {
        $this->num = $num;
    }

    public function sum()
    {
        $results = [];
        $sum = true;
        $nums = $this->separe($this->num);
        while ($sum) {
            if (count($nums)) {
                $result = array_sum($nums);
                if (count($this->separe($result)) == 1) {
                    $sum = false;
                }
                $nums = $this->separe($result);
                array_push($results, $result);
            } else {
                $sum = false;
            }
        }
        return $this->draw($results);
    }

    public function draw($items)
    {
        if (count($items)) {
            echo 'Resultados:<br>';
            echo '--------------------<br>';
            echo $this->num.'<br>';
            echo '--------------------<br>';
            foreach ($items as $item) {
                echo $item.'<br>';
                echo '--------------------<br>';
            }
        } else {
            return 'Nada que hacer';
        }
    }

    private function separe($num)
    {
        try {
            $nums = [];
            if ((int)$num == false) {
                throw new Exception('No es un número válido.');
            }
            $nums = str_split($num);
            if (!count($nums)) {
                return $num;
            }
            foreach ($nums as $k => $n) {
                $nums[$k] = (int)$n;
            }   
            return $nums;
        } catch (Exception $e) {
            echo $e->getMessage();
        }
    }
}

$num = new SumRecursive(9875);
$num->sum();