i've never really coded in php3, but my answer should still apply.
i'm going to assume its a user defined function (you created it).
are you sure you are uploading all the files of your project?
something you can do is copy the redirect() function and place it in the beggining of index.php3 file and see if it still gives you an error.
if it gives you an error, then i i'm out of ideas
if it doesn't give you an error, then you are not uploading the file which contain the redirect() function or your are not including it into the index.php3 file.