transfer list in textarea to comma delimited string

hi,
I have one textarea field in a registration form where visitor enters
keywords. even there is a not next to the field "please enter keywords
as comma delimited string", they enter as  a list, below each other.

I tried to convert the list into comma delimited string with several
solutions but non of them works:
$keywords = eregi_replace('\n', '.', $keywords);
$keywords = eregi_replace('\r', '.', $keywords);
$keywords = eregi_replace('\n\r', '.', $keywords);
$keywords = eregi_replace('<br>', '.', $keywords);

any help here?

thanks

-afan