php 随机生成一个字符串

/**
* create_rand随机生成一个字符串
* @param int $length 字符串的长度
* @param string $type 类型
* @return string
* @author:xjw129xjt(肖骏涛) xjt@ourstu.com
*/
function create_rand($length = 8, $type = 'all')
{
$num = '0123456789';
$letter = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
if ($type == 'num') {
$chars = $num;
} elseif ($type == 'letter') {
$chars = $letter;
} else {
$chars = $letter . $num;
}

$str = '';
for ($i = 0; $i < $length; $i++) {
$str .= $chars[mt_rand(0, strlen($chars) - 1)];
}
return $str;

}