Headers Already Sent - How To Overcome????

I am trying to do the following header ("Location: wlserver/index.php?userKey=$userKey&show=$userKey"); but I keep getting Warning: Cannot modify header information - headers already sent by ... The code on the header line above is not executing so I am assuming the previous headers being sent is causing this command to not execute. Part of the problem is that I am modifying software written by someone else to fit my needs. I do not want to make a lot of alterations because I don't want to pull my hair out when the next release shows up. Is there a command or someway to get around this problem without hacking out all of the other header sending code? I appreciate any help. Thank you. Mike