When users use classified ads site to post ads , for example like this one http://www.europegiant.com. Some people tend to over post the same ads many times . I know how I could avoid them from post the exact same ad twice , by checking if it already exists in the database .. but how can you still detect similarity in the ad if they tweak like once sentence from the ad and be able to reject it . I’d want to avoid duplicate content at all costs . How in PHP can I detect the similarity in the ads to 70% so I can reject spammers ?