Slack is printing raw JSON instead of styled message - php

Question

I am working on a Slack slash command in PHP that takes a Twitter username and returns that person's latest tweet.
I can't figure out how to display the user's profile image to make it look as nice as Slack's Twitter integration.
The issue it seems to me is that I have to include 'unfurl_media: true' in JSON that gets sent back to Slack. That means I can't just use echo to print out the Twitter data I want, which is in an associative array.
I tried grabbing the Twitter data I want from the associative array, encoding it again in JSON and then printing that. But all that does is print the JSON as plain text. I checked a JSON validator and it says what gets printed to Slack is valid JSON, so I don't understand why slack isn't converting it to a styled message.
Any suggestions?
Here's the code. It's frankencode pulled from a bunch of different places that I am trying to bend to my will.
<?php
require_once('TwitterAPIExchange.php');
$command = $_POST['command'];
$text = $_POST['text'];
$token = $_POST['token'];
if($token != '[insert your Slack slash command token here]'){
$msg = ":squirrel: The token for the slash command doesn't match. We're done here until IT fixes it. Don't worry, Squirrelock is on the case.";
die($msg);
echo $msg;
}
$settings = array(
'oauth_access_token' => "[insert access token here]",
'oauth_access_token_secret' => "[insert access token secret here]",
'consumer_key' => "[insert consumer key here]",
'consumer_secret' => "[insert consumer secret here]"
);
$url = "https://api.twitter.com/1.1/statuses/user_timeline.json";
$requestMethod = "GET";
$getfield = '?screen_name='.$text.'&count=1';
$twitter = new TwitterAPIExchange($settings);
$string = json_decode($twitter->setGetfield($getfield)
->buildOauth($url, $requestMethod)
->performRequest(), $assoc = TRUE);
foreach($string as $items)
{
$reply = "".$items['user']['profile_image_url']." *".$items['user'] ['name']."* ".$items['user']['screen_name']. "\n ".$items['text']."\n ".$items['created_at']."";
}
$data = json_encode(array(
"response_type" => "in_channel",
"text" => $reply,
"unfurl_media" => true,
"unfurl_links" => true
));
echo $data;
?>

Answer 1

Moving my comment down to an answer, since it seems to have fixed the problem:
Add header('Content-Type: application/json'); before echo $data;. The issue is that Slack is interpreting your response as text. It will interpret it as JSON only if the Content-Type is correct.