生成长度为4的字符串，字符包括abcdefg，怎么样得到所有的组合？

function dfs($pre, $chars, $arr, $lenArr) {
    if(!empty($pre) && in_array(strlen($pre), $lenArr)){ $arr[] = $pre; }
    if(!empty($chars)) {
        foreach ($chars as $char) {
            $tempChars = array();
            foreach ($chars as $c) {
                if ($c !== $char) { $tempChars[] = $c; }
            }
            $arr = $this->dfs($pre.$char, $tempChars, $arr, $lenArr);
        }
    }
    return $arr;
}

function get_combine() {
    $chars = array('a', 'b', 'c', 'd', 'e', 'f', 'g');
    $combineArray = array();
    $combineArray = $this->dfs('', $chars, $combineArray, array(4));
    echo count($combineArray).'
';
    var_dump($combineArray);
}