よく忘れがちなので、メモ。

// 現在日付からの加算

echo "1日後"   . date("Y/m/d", strtotime("1 day"  ));
echo "1週間後" . date("Y/m/d", strtotime("1 week" ));
echo "1ヶ月後" . date("Y/m/d", strtotime("1 month"));
echo "1年後"   . date("Y/m/d", strtotime("1 year" ));


// 指定日付からの加算

echo "1日後"   . date("Y/m/d", strtotime("2014/01/20 1 day"  ));
echo "1週間後" . date("Y/m/d", strtotime("2014/01/20 1 week" ));
echo "1ヶ月後" . date("Y/m/d", strtotime("2014/01/20 1 month"));
echo "1年後"   . date("Y/m/d", strtotime("2014/01/20 1 year" ));


// 現在日付からの減算

echo "1日前"   . date("Y/m/d", strtotime("-1 day"  ));
echo "1週間前" . date("Y/m/d", strtotime("-1 week" ));
echo "1ヶ月前" . date("Y/m/d", strtotime("-1 month"));
echo "1年前"   . date("Y/m/d", strtotime("-1 year" ));


// 指定日付からの減算

echo "1日前"   . date("Y/m/d", strtotime("2014/01/20 -1 day"  ));
echo "1週間前" . date("Y/m/d", strtotime("2014/01/20 -1 week" ));
echo "1ヶ月前" . date("Y/m/d", strtotime("2014/01/20 -1 month"));
echo "1年前"   . date("Y/m/d", strtotime("2014/01/20 -1 year" ));

文責：牧秀樹