<?php
function finddigits(string $s)
{
    $numbers = ["zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine"];
    $parsed = array_values(array_unique(str_split($s)));

    foreach ($numbers as $key => $value) {
        foreach (str_split($value) as $item) {
            if (!in_array($item, $parsed)) {
                unset($numbers[$key]);
            }
        }
    }

    return $numbers;
}

/**
Array
(
  [1] => one
  [2] => two
  [4] => four
)
 */
finddigits('owoftnuoer');