php json_decode returns NULL with syntax error

At the risk of duplication, here is my very own struggle with json_decode(). I've been through many if not all the posts on this and nothing has worked. So here goes:

Here is a sample of my json variable's content:

{"id":"212","game":"36","player":"1","currentChallenge":"3","startDate":"1456575719","endDate":"1456575918","attempts":"0","completed":"1"}

I checked it on http://jsonlint.com/ and it's good I am running these filters as per other forum suggestions:

$response = utf8_encode($response);
$response = str_replace('"', '"', $response);

And here is my decode line:

var_dump(json_decode($response, true));

I still get a NULL response and a syntax error (json_last_error()).

This data is coming from my own database and api. I tried inputting manually to the database and querying that, figuring I could get rid of any problems created by the input process. No luck there. And I have also sampled various tables to be sure it is not a problem related to the database setup.

And here is all my code:

if (isset($_POST['submit']) ) {

    $table = $_POST['table'];
    $id = $_POST['id'];
    $url = 'http://example.com/api.php/' . $table . '/' . $id;

    $client = curl_init($url);
    curl_setopt($client,CURLOPT_RETURNTRANSFER,true);

    $response = curl_exec($client);
    if ($response === false) {
        $info = curl_getinfo($curl);
        curl_close($curl);
        die('error occured during curl exec. Additioanl info: ' . var_export($info));
    }

    $response = utf8_encode($response);
    $response = str_replace('"', '"', $response);
    echo $response;

    echo '<br />Result of json_decode: '; 
    var_dump(json_decode($response, true));

    $json_errors = array(
        JSON_ERROR_NONE => 'No error has occurred',
        JSON_ERROR_DEPTH => 'The maximum stack depth has been exceeded',
        JSON_ERROR_CTRL_CHAR => 'Control character error, possibly incorrectly encoded',
        JSON_ERROR_SYNTAX => 'Syntax error',
    );
    echo 'Last error : ', $json_errors[json_last_error()], PHP_EOL, PHP_EOL;

}

Thank you!

Copyright Notice：Content Author:「user2073143」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/35781163/php-json-decode-returns-null-with-syntax-error