You may also want to just use arrays, and let PHP handle the JSON encoding for you. This particular error is common if the JSON is invalid for some reason. So, for example, you could set your parameters like this:
$params = array(
"key" => "keyhere",
"message" => array(
"html" => $content,
"text" => $content_text,
"to" => array(
array("name" => $to, "email" => $to)
),
"from_email" => $from,
"from_name" => $from,
"subject" => $subject,
"track_opens" => true,
"track_clicks" => true
),
"async" => false
);
$postString = json_encode($params);
You can also use json_decode
to parse the response if needed.