mysql timestamp output as formatted Date not working - php

Question

I have a crazy phenomenon in my php script. I have defined a column as a timestamp in a mysql table. I fill it with the function:
date("Y-m-d H:i:s");
The data in the table then look like this: 2017-04-19 17:08:45
When I query this column with mysqli as a unix timestamp again:
SELECT UNIX_TIMESTAMP (timestamp) as timestampUnix FROM posts
Then binding the result using bind_result to the variable $timestampUnix.
I can echo the variable with
echo $timestampUnix;
and it outputs a correct timestamp like this: 1492614559
however if i do the following:
$timestampUnix2 = $timestampUnix;
echo $timestampUnix2;
there is simply no echo output... What is the reason?
I tried this because I actually want echo only the date in an other format with:
date('d.m.Y', $timestampUnix)
and it gave me 01.01.1970 and i wondered why the timestamp must be 0 but it isnt since when i directly echo it it gives me a correct one.
however when i do
Date('d.m.Y', 1492614559)
it gives me the correct date.. no clue what is going on there!
i know there are many other questions about mysql php Date output, but no one have this issue as i think i cannot do something with the variable i got from the query.
thanks in advance!
edit: i attach the complete code in question:
---the query that inputs the data in the db----
$timestamp = date("Y-m-d H:i:s");
mysqli_query($mysqli,"INSERT INTO posts (timestamp)
VALUES ('$timestamp')");
---the query that fetches the data----
$results = $mysqli->prepare("SELECT UNIX_TIMESTAMP(timestamp) as timestampUnix FROM posts");
$results->execute(); //Execute prepared Query
$results->bind_result($timestampUnix); //bind variables to prepared statement
$postdate = date('d.m.Y',$timestampUnix)
echo $postdate;