Php strtotime "rounding" end of month

Any idea why strtotime sometimes round the end of the month to the next month?

for($i=1;$i<12;$i++)
{
    $d = "Y-$i-t 00:00:00";
    echo "Date as String: " . $d = date($d) . "\n";
    echo "Date as UnixTime: " . strtotime($d) . "\n";
    echo "UnixTime back to String: " . $d = date('Y-m-d 00:00:00', strtotime($d)) . "\n";
    echo "String back to UnixTime: " . strtotime(开发者_开发百科$d) . "\n\n";
}

Results:

Date as String: 2011-1-31 00:00:00

Date as UnixTime: 1296450000

UnixTime back to String: 2011-01-31 00:00:00

String back to UnixTime: 1296450000

Date as String: 2011-2-31 00:00:00

Date as UnixTime: 1299128400

UnixTime back to String: 2011-03-03 00:00:00

String back to UnixTime: 1299128400

Date as String: 2011-3-31 00:00:00

Date as UnixTime: 1301544000

UnixTime back to String: 2011-03-31 00:00:00

String back to UnixTime: 1301544000

Date as String: 2011-4-31 00:00:00

Date as UnixTime: 1304222400

UnixTime back to String: 2011-05-01 00:00:00

String back to UnixTime: 1304222400

Date as String: 2011-5-31 00:00:00

Date as UnixTime: 1306814400

UnixTime back to String: 2011-05-31 00:00:00

String back to UnixTime: 1306814400

Date as String: 2011-6-31 00:00:00

Date as UnixTime: 1309492800

UnixTime back to String: 2011-07-01 00:00:00

String back to UnixTime: 1309492800

Date as String: 2011-7-31 00:00:00

Date as UnixTime: 1312084800

UnixTime back to String: 2011-07-31 00:00:00

String back to UnixTime: 1312084800

Date as String: 2011-8-31 00:00:00

Date as UnixTime: 1314763200

UnixTime back to String: 2011-08-31 00:00:00

String back to UnixTime: 1314763200

Date as String: 2011-9-31 00:00:00

Date as UnixTime: 1317441600

UnixTime back to String: 2011-10-01 00:00:00

String back to UnixTime: 1317441600

Date as String: 2011-10-31 00:00:00

Date as UnixTime: 1320033600

UnixTime back to String: 2011-10-31 00:00:00

String back to UnixTime: 1320033600

Date as String: 2011-11-31 00:00:00

Date as UnixTime: 1322715600

UnixTime back to String: 2011-12-01 00:00:00

String back to UnixTime: 1322715600

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t returns the number of days in the month of the date that was passed (or, in your case, since you're not passing a date, the current month). Because you are running this code in January, the result will be 31.

February doesn't have 31 days so PHP will add the extra 3 days moving you to March 3rd.

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Because not all months have 31 days.

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30 days has September, April, June, and November. All the rest have 31, except February which has 28 and sometimes 29.