check if email exist before new registration

hi i need to check if email exist before new user registration .
i have following
$strSQLExists = "select email from user where email='".$userdata["email"]."'";

$rsExists = db_query($strSQLExists,$conn);

$data=db_fetch_array($rsExists);

if($data)

{

return false;

}

else

{
return true;

}
but get following error
mysqli_query() expects parameter 1 to be mysqli, null given