需要在指定日期添加指定的天数

Need to add a specified numbers of days to a selected date. Not the current date. Im using strtotime in this fashion:

$date = $_POST['r_date'];
$r_date = $date;


$txts_rental = $_POST['txts_rental'];
  switch ($txts_rental){
      case "250.00":
          $s_length = "30";
             break;
    case "575.00":
          $s_length = "90";
             break;
    case "975.00":
          $s_length = "180";
             break;
    case "1200.00":
          $s_length = "365";
             break;
  }

 $rn_date = date( "Y-m-d", strtotime( "$s_length day" ) );

the date( "Y-m-d" needs to be the the $r_date variable in order to get the renewal date from the inputted date

You can use

$EndDateTime = DateTime::createFromFormat('d/m/Y', "16/07/2017");
$EndDateTime->modify('+6 days');
echo $EndDateTime->format('d/m/Y');

or

$today = "2015-06-15"; // Or can put $today = date ("Y-m-d");
$fiveDays = date ("Y-m-d", strtotime ($today ."+5 days"));

You need to change your method as below. add the target date in second parameter so that the date not added in current date

$date = $_POST['r_date'];
$r_date = $date;


$txts_rental = $_POST['txts_rental'];
  switch ($txts_rental){
      case "250.00":
          $s_length = "30";
             break;
    case "575.00":
          $s_length = "90";
             break;
    case "975.00":
          $s_length = "180";
             break;
    case "1200.00":
          $s_length = "365";
             break;
  }

 $rn_date = date( "Y-m-d", strtotime( "$date +".$s_length." day" ) );