replace into .. values ... where???

[sabret00the]

i'm trying to do this [sql] $DB_site->query("
REPLACE INTO prs_users(favourites)
VALUES(if(favourites='', $p, concat_ws(' ', favourites, $p)))
WHERE userid = $bbuserinfo[userid]
");[/sql]

but it gives me this error

mysql error: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'WHERE userid = 10' at line 3

mysql error number: 1064

not sure if i'm doing a good job of explaining this so let me know if not and i'll try harder.