php possible datetime gotcha

Initially, i thought it was a bug in php...

but i think thats how its suppose to work.

If any month with 31days, and the last month has less days than current date,

the strtotime("-1 month", mktime())

will return this month date, instead of last month.

Example: strtotime("-1 month", strtotime("2011-07-01"))

results: time of 2011-06-01.

But,

Example: strtotime("-1 month", strtotime("2011-07-31"))

results: time of 2011-07-01.

This is because minus 1 month of 2011-07-31 in math => 2011-06-31,

which results to 2011-07-01 due to no 31 days in june.