Space in php generated link....

Hi, I am getting some values from the database and according to that building a link to modify the table via another php script. This is the main line down: <a href=modify.php?n=".$row[3]."&u=".$row[17]."&p=".$row[18].">Modify</a> $row[3] is for the "name" the others are not really important my problem is when the name is "thegreatname" its ok, but when there the firstname,lastname are entered like "Great Name" it gives me trouble.... when i go to click on the created link on top only "Great" shows eg: http://blahsite.com/php/modify.php?n=Great the other variables of "u" and "p" (username/pass) are not showing.... What can i do? Kindly help, -Ryan