var_dump says String of 75 characters, can't echo it and doesn't json_decode()

Question

I am posting some data to a PHP page with AJAX, but I can't decode it. json_decode() returned NULL. So I figured that if I'd echo it I could run it through jsonlint. But nothing showed up. To find out if the data reaches the PHP page I tried to var_dump() the data. Strange enough it showed:

string(75) "{\"title\":\"fds\",\"body\":\"fds\",\"tags\":\"fds\",\"type\":\"question\"}"

So the data reaches the page. I ran it through jsonlint after removing the backslashes and it said the JSON was valid. How is this possible?

I got a valid JSON string of 75 characters according to var_dump() and jsonlint, but I can't echo it or json_decode() it.

EDIT:
Apparently I did something wrong, I can echo it now (I probably made a typo in the $_POST array key). It echoes this:

{\"title\":\"tre\",\"body\":\"tre\",\"tags\":\"tre\",\"type\":\"question\"}

Once again, without backslashes JSONLint tells me it is valid JSON.

EDIT 2:
Here's the PHP code:

// code to check if the user is logged in
$post = json_decode($_POST['q']);
echo "var_dump(\$_POST['q']): ";
var_dump($_POST['q']);
echo "<BR /><BR />";
echo "var_dump(\$post): ";
var_dump($post);    

Ouput:

var_dump($_POST['q']): string(74) "{\"title\":\"gfd\",\"body\":\"gf\",\"tags\":\"gfd\",\"type\":\"question\"}" 

var_dump($post): NULL

JS code:

var simpleObj = {title: "rew", body: "rew", tags: "rew", type: "question"};
$.post('savepost.php', "q=" + JSON.stringify(simpleObj), function(data) {
    $('#resultDiv').html("DATA: " + data);
});

Final edit:
I finally solved it myself. I actually discovered the problem even before I asked, but thought it was 'harmless' and didn't think that could cause this problem.

Answer 1

I found out the answer! When I ran it through jsonlint.com it said it was invalid because of the backslashes. I thought these were just there to mark the quotes in the JSON as normal quotes, not string ending and starting quotes and I thought they were 'harmless'.
I then thought of the function addslashes(), and figured there should be a reversed one too. I found stripslashes() and that solved the problem.

New code:

// code to check if the user is logged in
$post = json_decode(stripslashes($_POST['q']));
echo "var_dump(\$_POST['q']): ";
var_dump($_POST['q']);
echo "<BR /><BR />";
echo "var_dump(\$post): ";
var_dump($post);