编写一个函数GeneratePassword，它接受两个参数，一个整数和一个由字母（a-z）和数字（0-9）组成的字符串。
当调用GeneratePassword(5,'abc0123')时，它应该返回一个从"abc0123"中提取的5个字符的随机字符串。
例如：GeneratePassword(7,'abczxc012394')可以返回以下任何输出：
2c00acb
2c23z93
030b2a4
我想你在找作业标签。
本着帮助他人的精神，我将发布一个有评论的解决方案。但是，请记住，变得更好的唯一方法是先尝试，然后再提问。也就是说，尝试一下，然后问别人你哪里出了问题。
示例/演示：
/**
* Generate a password N characters long consisting of characters
*
* @param int $size
* @param string $characters
* @param callback $random (optional) source of random, a function with two parameters, from and to
* @return string|NULL password
*/
function generate_password($size, $characters, $random = 'rand') {
// validate $size input
$size = (int) $size;
if ($size <= 0) {
trigger_error(sprintf('Can not create a password of size %d. [%s]', $size, __FUNCTION__), E_USER_WARNING);
return NULL;
}
if ($size > 255) {
trigger_error(sprintf('Refused to create a password of size %d as this is larger than 255. [%s]', $size, __FUNCTION__), E_USER_WARNING);
return NULL;
}
// normalize $characters input, remove duplicate characters
$characters = count_chars($characters, 3);
// validate number of characters
$length = strlen($characters);
if ($length < 1) {
trigger_error(sprintf('Can not create a random password out of %d character(s). [%s]', $length, __FUNCTION__), E_USER_WARNING);
return NULL;
}
// initialize the password result
$password = str_repeat("'x00", $size);
// get the number of characters minus one
// your string of characters actually begins at 0 and ends on the
// string-length - 1:
// $characters[0] = 'a'
// $characters[1] = 'b'
// $characters[2] = 'c'
$length--;
// get one random character per each place in the password
while ($size--)
{
// generate a random number between 0 and $length (including)
$randomValue = $random(0, $length);
// that random number is used to turn the number into a character
$character = $characters[$randomValue];
// set the random character
$password[$size] = $character;
}
// return the result
return $password;
}