求一个一维数组中，n个元素的不同组合,并返回该数组

<code>$data = array('a1','b2','c3','d4','e5','x1','y1','z1');

foreach ($data as $k_1 => $v_1) {
    foreach ($data as $k_2 => $v_2) {
        foreach ($data as $k_3 => $v_3) {
            if ($v_1 !== $v_2 && $v_1 !== $v_3 && $v_2 !== $v_3) {
                var_dump([$v_1,$v_2,$v_3]);
            }
        }    
    }
}</code>

<code><?php $arr = array('a','b','c','d');
$result = array();
$t = getCombinationToString($arr, 3);
print_r($t);

function getCombinationToString($arr, $m) {
    if ($m ==1) {
        return $arr;
    }
    $result = array();

    $tmpArr = $arr;
    unset($tmpArr[0]);
    for($i=0;$i<count($arr);$i++) {
        $s = $arr[$i];
        $ret = getCombinationToString(array_values($tmpArr), ($m-1));

        foreach($ret as $row) {
            $result[] = $s . $row;
        }
    }

    return $result;
}
?>
</code>

<code>[0] => abc
[1] => abd
[2] => acc
[3] => acd
[4] => adc
[5] => add
[6] => bbc
[7] => bbd
[8] => bcc
[9] => bcd
[10] => bdc
[11] => bdd
[12] => cbc
[13] => cbd
[14] => ccc
[15] => ccd
[16] => cdc
[17] => cdd
[18] => dbc
[19] => dbd
[20] => dcc
[21] => dcd
[22] => ddc
[23] => ddd</code>