function week_start_date($wk_num, $yr, $first = 1, $format = 'l, d-M-y') {
    $wk_ts = strtotime('+' . $wk_num . ' weeks', strtotime($yr . '0101'));
    $mon_ts = strtotime('-' . date('w', $wk_ts) + $first . ' days', $wk_ts);
    return date($format, $mon_ts);
}



function date_get_week_start($date, $date_format = "Y-m-d")
{
    /*
     * Week Start from Monday
     */
    $year = date('Y', strtotime($date));
    $week = date('W', strtotime($date));

    $date = week_start_date($week, $year, 1, $date_format);

    return $date;
}

function date_get_week_end($date, $date_format = "Y-m-d")
{
    /*
     * Week Ends on Sunday
     */
    $year = date('Y', strtotime($date));
    $week = date('W', strtotime($date));

    $date = week_start_date($week, $year, 7, $date_format);

    return $date;
}

function date_get_month_start($date, $date_format = "Y-m-d")
{
    $year = date('Y', strtotime($date));
    $month = date('m', strtotime($date));
    $date = $year."-".$month."-"."01";
    return date($date_format, strtotime($date));
}

function date_get_month_end($date, $date_format = "Y-m-d")
{
    $year = date('Y', strtotime($date));
    $month = date('m', strtotime($date));
    return date($date_format,strtotime('-1 second',strtotime('+1 month',strtotime($month.'/01/'.$year.' 00:00:00'))));
}

function date_get_year_start($date, $date_format = "Y-m-d")
{
    $year = date('Y', strtotime($date));
    $date = $year."-01-01";
    return date($date_format, strtotime($date));
}

function date_get_year_end($date, $date_format = "Y-m-d")
{
    $year = date('Y', strtotime($date));
    $date = $year."-12-31";
    return date($date_format, strtotime($date));
}

These are some date functions. 

Get Current Week Start Date
Get Current Week End Date
Get Current Month Start
Get Current Month End
Get Current Year Start
Get Current Year End

and these functions can help you getting last week's start date or next month end date. 

To get last week's start date, remove 7 days from current week, and pass that date to get current week start date and it will return last week's start date.

Some Date Functions

date

PHP