好吧，您并没有阐明我在评论中提出的所有问题，但是我假设您想要一个可以使用字符串可能的"字符和要返回的字符串长度.为了清楚起见，按要求进行了彻底注释，使用了比我通常更多的变量:

Well, you didn't clarify all the questions I asked in my comment, but I'll assume that you want a function that can take a string of "possible" characters and a length of string to return. Commented thoroughly as requested, using more variables than I would normally, for clarity:

function get_random_string($valid_chars, $length)
{
    // start with an empty random string
    $random_string = "";

    // count the number of chars in the valid chars string so we know how many choices we have
    $num_valid_chars = strlen($valid_chars);

    // repeat the steps until we've created a string of the right length
    for ($i = 0; $i < $length; $i++)
    {
        // pick a random number from 1 up to the number of valid chars
        $random_pick = mt_rand(1, $num_valid_chars);

        // take the random character out of the string of valid chars
        // subtract 1 from $random_pick because strings are indexed starting at 0, and we started picking at 1
        $random_char = $valid_chars[$random_pick-1];

        // add the randomly-chosen char onto the end of our string so far
        $random_string .= $random_char;
    }

    // return our finished random string
    return $random_string;
}

要使用示例数据调用此函数，应将其命名为:

To call this function with your example data, you'd call it something like:

$original_string = 'abcdefghi';
$random_string = get_random_string($original_string, 6);

请注意，此函数不会检查传递给它的有效字符中的唯一性.例如，如果使用有效的'AAAB'字符字符串进行调用，则为每个字母选择A作为B的可能性将增加三倍.根据您的需要，可以将其视为错误或功能.

Note that this function doesn't check for uniqueness in the valid chars passed to it. For example, if you called it with a valid chars string of 'AAAB', it would be three times more likely to choose an A for each letter as a B. That could be considered a bug or a feature, depending on your needs.