Calling Watson API from PHP code

I am trying to call Natural Language Understanding Watson API from my PHP code using curl. I have successfully tried curl from terminal. It gives some result on executing this command:

curl -u "my_username":"my_password" "https://gateway.watsonplatform.net/natural-language-understanding/api/v1/analyze?version=2017-02-27&text=I%20still%20have%20a%20dream.&features=sentiment,keywords"

However when I try to use curl in php using this code:

<?php
$url = "https://gateway.watsonplatform.net/natural-language-understanding/api/v1/analyze?version=2017-02-27&text=I%20still%20have%20a%20dream%2C%20a%20dream%20deeply%20rooted%20in%20the%20American%20dream%20%E2%80%93%20one%20day%20this%20nation%20will%20rise%20up%20and%20live%20up%20to%20its%20creed%2C%20%22We%20hold%20these%20truths%20to%20be%20self%20evident%3A%20that%20all%20men%20are%20created%20equal.&features=sentiment,keywords";
$post_args = array(
         'version' => '2017-02-27',
         'url' => 'https://davidwalsh.name/curl-post',
         'features' => 'sentiment,keywords' 
     );
$headers = array(
        'Content-Type:application/json'
    );
$ch = curl_init();
curl_setopt($ch, CURLOPT_USERPWD, "my_username:my_password");
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
//curl_setopt($curl, CURLOPT_HTTPAUTH, CURLAUTH_BASIC);
//curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
//curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query($post_args));
$result = curl_exec($ch);
    if ($result === FALSE) {
        die('Send Error: ' . curl_error($ch));
    }
curl_close($ch);
echo $result;
?>

I get this result:

{
"error": "unsupported media type",
"code": 415
}

What should be done?

I can't believe I was stuck here due to a blank space. Replacing this:

$headers = array(
        'Content-Type:application/json'
    );

with

$headers = array(
        'Content-Type: application/json'
    );

worked. Note the blank space between : and application/json.