'; // Test input if (!ctype_digit ($input) || // only digits $input > PHP_INT_MAX || // <= MAX INT size strlen (count_chars ($input, 3)) < 2) { // min. 2 different digits exit ('Invalid Input'); } for ($i=0;;$i++) { $arr_a[] = str_sort ($i ? $arr_res[$i-1]: $input); $arr_d[] = strrev ($arr_a[$i]); $arr_res[] = (string) $arr_d[$i] - $arr_a[$i]; // Add leading zero if necessary while (strlen ($arr_res[$i]) < $length) { $arr_res[$i] = '0'.$arr_res[$i]; } // print calculation echo $i+1, ': ', $arr_d[$i], ' - ', $arr_a[$i], ' = ', $arr_res[$i], '
'; // Test if Kaprekar's constant was found. if ($i>0 && $arr_res[$i] == $arr_res[$i-1]) { echo "Found Kaprekar's constant!
"; break; } // Test if a sequence was found. $r = array_search ($arr_res[$i], $arr_res); if ($r < $i) { echo "Found sequence!
"; echo "Sequence is ... "; for ($j=$r; $j<=$i; $j++) { echo $arr_res[$j], " - "; } echo "...
"; break; } // Give up after 100 iterations... // Perhaps not nessery. if ($i > 99) { echo '...give up...
'; break; } } // Sort string in ascending order. function str_sort (string $str) { $arr = str_split ($str); sort ($arr); return implode ($arr); } ?>

kaprekar_constant.php