[http] Should I URL-encode POST data?

I'm POSTing data to an external API (using PHP, if it's relevant).

Should I URL-encode the POST variables that I pass?

Or do I only need to URL-encode GET data?

UPDATE: This is my PHP, in case it is relevant:

$fields = array(
    'mediaupload'=>$file_field,
    'username'=>urlencode($_POST["username"]),
    'password'=>urlencode($_POST["password"]),
    'latitude'=>urlencode($_POST["latitude"]),
    'longitude'=>urlencode($_POST["longitude"]),
    'datetime'=>urlencode($_POST["datetime"]),
    'category'=>urlencode($_POST["category"]),
    'metacategory'=>urlencode($_POST["metacategory"]),
    'caption'=>($_POST["description"])
);
$fields_string = http_build_query($fields);
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL,$url);
curl_setopt($ch,CURLOPT_POST,count($fields));
curl_setopt($ch,CURLOPT_POSTFIELDS,$fields);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
$response = curl_exec($ch);

Above posts answers questions related to URL Encoding and How it works, but the original questions was "Should I URL-encode POST data?" which isn't answered.

From my recent experience with URL Encoding, I would like to extend the question further. "Should I URL-encode POST data, same as GET HTTP method. Generally, HTML Forms over the Browser if are filled, submitted and/or GET some information, Browsers will do URL Encoding but If an application exposes a web-service and expects Consumers to do URL-Encoding on data, is it Architecturally and Technically correct to do URL Encode with POST HTTP method ?"

@DougW has clearly answered this question, but I still like to add some codes here to explain Doug's points. (And correct errors in the code above)

Solution 1: URL-encode the POST data with a content-type header :application/x-www-form-urlencoded .

Note: you do not need to urlencode $_POST[] fields one by one, http_build_query() function can do the urlencoding job nicely.

$fields = array(
    'mediaupload'=>$file_field,
    'username'=>$_POST["username"],
    'password'=>$_POST["password"],
    'latitude'=>$_POST["latitude"],
    'longitude'=>$_POST["longitude"],
    'datetime'=>$_POST["datetime"],
    'category'=>$_POST["category"],
    'metacategory'=>$_POST["metacategory"],
    'caption'=>$_POST["description"]
);

$fields_string = http_build_query($fields);

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL,$url);
curl_setopt($ch, CURLOPT_POST,1);
curl_setopt($ch, CURLOPT_POSTFIELDS,$fields_string);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
$response = curl_exec($ch);

Solution 2: Pass the array directly as the post data without URL-encoding, while the Content-Type header will be set to multipart/form-data.

$fields = array(
        'mediaupload'=>$file_field,
        'username'=>$_POST["username"],
        'password'=>$_POST["password"],
        'latitude'=>$_POST["latitude"],
        'longitude'=>$_POST["longitude"],
        'datetime'=>$_POST["datetime"],
        'category'=>$_POST["category"],
        'metacategory'=>$_POST["metacategory"],
        'caption'=>$_POST["description"]
    );

    $ch = curl_init();
    curl_setopt($ch, CURLOPT_URL,$url);
    curl_setopt($ch, CURLOPT_POST,1);
    curl_setopt($ch, CURLOPT_POSTFIELDS,$fields);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
    $response = curl_exec($ch);

Both code snippets work, but using different HTTP headers and bodies.

curl will encode the data for you, just drop your raw field data into the fields array and tell it to "go".