$str = "www.MysqL.com";
echo implode('.',array_slice(explode('.',$str),2)); // prints "www.MysqL"
echo implode('.',-2));   // prints "MysqL.com"

轻松将其转换为功能：

function substring_index($subject,$delim,$count){
    if($count < 0){
        return implode($delim,array_slice(explode($delim,$subject),$count));
    }else{
        return implode($delim,$count));
    }
}