strtotime() Get 7th day of -1 month

Question

I have tried :

$firstOfMonth = "2015-01-01";
$last_month = date("Y-m-d", strtotime('first day of -1 month', strtotime($firstOfMonth)));
                                    /* ^^^^^ This gives me 01 of last month */
/*** Tried 'seventh day of -1 month'
/*   it gives 1970-01-01
*/

What I want is to get the 07 of last month.

Answer 1

uhm....the first day of a month is allways 01....so basically you only need year and month:

$firstOfMonth = "2015-01-01";
$last_month = date("Y-m-01",strtotime("-1 month",strtotime($firstOfMonth)));

Answer 2

Just like Mr.Manhattan said, you just need the year and month and to the seventh day you can leave it hard coded or in a variable:

date("Y-m-07",strtotime("-1 month"));
$fixedDay = '07';
echo date("Y-m-$fixedDay",strtotime("-1 month"));