Calculate number of days in PHP

Hi I am trying to basically, store current date, get the date of when user registered and compare to set number of days which is 7
See code below - basically, the user is allowed 7 days to complete something. What I want to do is, Set the amount of days to 7 and then this can be compared with the dates so it outputs how many days they have left..so when they first go onto the web page, itl start of with 7 and countsdown the days 6,5,4,3,2,1... each day

Can someone help with the below?

<?php

include ("dbConnect.php");
$UserID =$_SESSION['currentUserID'];

$dbQuery=$conn->prepare("select Date from Results where UserID=$UserID");
$dbQuery->execute();

  while($dbRow = $dbQuery->fetch(PDO::FETCH_ASSOC)) {
  $StartDate=$dbRow['Date'];
   $CurrDate= date("Y-m-d");   
    $Days=7;
-added 
function dayCount($CurrDate, $StartDate) {
    $first_date = strtotime($StartDate);
    $second_date = strtotime($CurrDate);
    $days_diff = $second_date - $first_date;
    return date('d',$days_diff);
}

 }