PHP生成随机字符串与唯一字符串

<?php
/*
 * 生成随机字符串
 * @param int $length 生成随机字符串的长度
 * @param string $char 组成随机字符串的字符串
 * @return string $string 生成的随机字符串
 */
function str_rand($length = 32, $char = ‘0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ‘) {
    if(!is_int($length) || $length < 0) {
        return false;
    }

    $string = ‘‘;
    for($i = $length; $i > 0; $i--) {
        $string .= $char[mt_rand(0, strlen($char) - 1)];
    }

    return $string;
}

echo str_rand(),"<br />";

/*
 * 生成32位唯一字符串
 */
$uniqid = md5(uniqid(microtime(true),true));
echo $uniqid;