Error in PHP $date2 = $date1->format('Y-m-d') - php

Question

I get the following error:
PHP Fatal error: Call to a member function format() on a non-object
in code:
$date = new DateTime();
$date1 = $date->modify('-6 months');
$date2 = $date1->format('Y-m-d');
I want to get this date of 6 months before from now and delete all entries in database which are earlier than this 6 months date:
$query = $conn->prepare("DELETE FROM files WHERE files.date < ?");
$query->bind_param('s', $date2);
$query->execute();
In MySQL "date" field is in files table of datatype "timestamp" whose value is "CURRENT_TIMESTAMP" stored by MySQL as default when a row is created.

Answer 1

This code get you 6 month ago from now:
date('Y-m-d', strtotime('now -6 month'))
EDIT:
and use DateTime:
echo (new DateTime('-6 months'))->format('Y-m-d');

Answer 2

try below code:
<?php
$date = new DateTime('-6 months');
echo $date2 = $date->format('Y-m-d');
?>

Answer 3

It appears you are using an outdated version of php.
Starting with php 5.3+ DateTime::modify returns the datetime object is was called upon prior to that null was returned.
You don't need to assign the result of modify to a new variable as it modifies the current object and does not return a new one.
To make it work on 5.2:
<?php
$date = new DateTime();
$date->modify('-6 months');
$dateS = $date->format('Y-m-d');
var_dump($dateS);
You should however update your php to a supported version.
PHP 5.4+ short-example:
$dateS = (new DateTime('-6 months'))->format('Y-m-d');

Answer 4

Seems like the call to
$date->modify('-6 months');
encounters an error and does not return a DateTime instance but probably false (see documentation).
Which PHP version are you using? I could not reproduce that error with the given example on PHP 5.5.9
Maybe you could just use the strtotime function
EDIT:
I took the Wayback Machine from Internet Archive and found that older versions of PHP ( i.e. 5.1.0) returned NULL instead of the DateTime object: see for yourself. In that case it seems like you apply the modification on your date directly so you need to change your code to
$date = new DateTime();
$date->modify('-6 months');
$dateString = $date->format('Y-m-d');
$query = $conn->prepare("DELETE FROM files WHERE files.date < ?");
$query->bind_param('s', $dateString);
$query->execute();