Date difference in PHP code does not execute - php

Question

I have a code in PHP 5.5.11 where I am trying to do the following:
Get today's date in a variable --> $today
Calculate the end of month from a date in a form --> $st_dt_eom
if difference between these 2 dates is more than 5 days then execute a code. The code in the if condition below does not execute.
$today= date();
if($_POST['Submit']=='SAVE')
{
$st_dt=YYYYMMDD($_POST['st_dt'],"-");
$st_dt_eom= datetime::createfromformat('YYYYMMDD',$st_dt);;
$st_dt_eom->modify('last day of this month');
$diff = $today->diff($st_dt_eom);
$diffDays= intval($diff->format("%d")); //to get integer number of days
if($diffDays>5){
redirect("index.php");
}
}

Answer 1

An example:
// 2022-12-19
$today = date('Y-m-d');
// $_POST['st_dt']
$st_dt = '2022-12-31';
function dateDiffDays($today, $st_dt)
{
$today_obj= DateTime::createfromformat('Y-m-d', $today);
$st_dt_eom= DateTime::createfromformat('Y-m-d', $st_dt);
$diff = $today_obj->diff($st_dt_eom);
return $diff->days;
}
// int(12)
$res = dateDiffDays($today, $st_dt);
use var_dump to locate your bug.

Answer 2

A few suggestions to improve your code and produce something workable:
<?php
// Instead of using date(), use a DateTime() then you're comparing two DateTimes later on
$today = new DateTime();
// I'm assuming that your YYYYMMDD function removes "-" from the $_POST['st_dt']
// to provide a date in the format YYYYMMDD (or Ymd in PHP).
// Unfortunately, DateTime doesn't understand that format.
// So I'd change this for keeping Y-m-d (YYYY-MM-DD).
// Or modify your code to return that format!
$st_dt = $_POST['st_dt'];
// Watch out for your cAsE when using PHP functions!
$st_dt_eom = DateTime::createFromFormat('Y-m-d',$st_dt);
$st_dt_eom->modify('last day of this month');
$diff = $today->diff($st_dt_eom);
$diffDays= intval($diff->format("%d"));
if($diffDays > 5){
redirect("index.php");
}