select where true, or select where input = '$var'

i want to select based on input, but if input is not provided or if
input is empty, then i want to select all rows.

1 select *
2 from table
3 if input = '' then
4  where true
5 else
6  where input = '$sanitized_variable'
7 end if;
(syntax error at 3)

i also looked at 'case' but i don't think it applies here.

http://www.postgresql.org/docs/9.3/static/functions-conditional.html
http://dba.stackexchange.com/questions/41067/getting-select-to-return-a-constant-value-even-if-zero-rows-match