How to INSERT based on a condition?

I have these INSERT query:

$db->prepare("INSERT INTO
              events (post_id, table_code, other_id, user_id, author_id, date_time )
              VALUES (?      , 15        , ?       , ?      , ?        , UNIX_TIMESTAMP()),
                     (?      , 15        , ?       , ?      , ?        , UNIX_TIMESTAMP())
")->execute(array($answer_id, $answer_id, $author_ques_id, $author_ques_id,
                  $answer_id, $answer_id, $author_ques_id, $author_ans_id));

As you see, query above inserts two rows into events table. Now I need to put a condition on the way of second row. I mean if $condition is true then inserts second rows, else it shouldn't insert second row. (first row always should be inserted).

Note: $condition always is containing a boolean value.

Well, how can I put that condition on the way of second row inserting?