<?php
function card_deal($player_nums, $cards) {
    if ($player_nums == 0) {
        exit("プレイヤーが0人です");
    }
    $cards_ary = str_split($cards);
    $length = count($cards_ary);

    $cards = array();   // return値
    if ($player_nums > $length) {
        for ($j = 0; $j < $player_nums; $j++) {
            $cards[$j] = "";
        }
    } else {
        $length = $length - ($length % $player_nums);
        // n人目のカードを求める
        for ($n = 0; $n < $player_nums; $n++) {
            for ($i = 0; $i < $length; $i++) {
                if (($i % $player_nums) == $n) {
                    $cards[$n] .= $cards_ary[$i];
                }
            }
        }
    }
    return $cards;
}

var_dump(card_deal(3, "123123123"));
var_dump(card_deal(4, "123123123"));
var_dump(card_deal(6, "01234"));
var_dump(card_deal(0, "123456789"));
?>

array(3) {
  [0]=>
  string(3) "111"
  [1]=>
  string(3) "222"
  [2]=>
  string(3) "333"
}
array(4) {
  [0]=>
  string(2) "12"
  [1]=>
  string(2) "23"
  [2]=>
  string(2) "31"
  [3]=>
  string(2) "12"
}
array(6) {
  [0]=>
  string(0) ""
  [1]=>
  string(0) ""
  [2]=>
  string(0) ""
  [3]=>
  string(0) ""
  [4]=>
  string(0) ""
  [5]=>
  string(0) ""
}
プレイヤーが0人です