PHP随机位置向字符串插入关键字

$times = 4; //关键字出现频率
$keyword = "【fuck】"; //要替换的关键字
$str = "棕是呆硫酸铝中伙右中西医三阿拉山口地方就看到las决哦iasfls.sdmflaskdf洛杉矶的法律卡死噢iuoi020990拉开时间的发生了疯狂";
$strlen = mb_strlen($str, "gb2312");
//随机生成插入位置，根据$times决定插入频率
for ( $i = 0; $i < $times; $i ++ )
{
$arr[] = mt_rand(0, $strlen);
}
$arr = array_unique($arr); //过滤数组重复元素
sort($arr); //对数组重新排序，按照值由小到大
//开始替换
$i = 0;
$str_new = "";
foreach( $arr as $v )
{
$str_new .= mb_substr($str, $i, $v - $i, "gb2312") . $keyword;
$i = $v;
}
$str_new .= mb_substr($str, $i, $strlen - $i, "gb2312");
echo($str_new);