Parameters missing/damaged in PHP curl request - php

Question

Update: I have solved the problem by using file_get_contents (or other file operations such as fopen) instead of curl, as is shown in this example: https://developers.google.com/chart/image/docs/post_requests. The question is, therefore, why curl is not able to POST the correct parameters while file operations can? This is essentially asking the difference between curl and file_get_contents...
The original problem
I'm using curl in PHP to download an image. The image is a Google Image Chart (https://developers.google.com/chart/image/) generated according to parameters passed through GET or POST to http://chart.googleapis.com/chart. Here is an example chart:
http://chart.googleapis.com/chart?cht=p3&chd=t:60,40&chs=250x100&chl=Hello|World
The simple way is to GET this URL, and it works. However when I tried to GET a longer URL (with more parameters for the data of the chart), Google Chart returned a 400 page saying "malformed or illegal request". I pointed my browser to that longer URL and saw the desired chart, and thus could verify that the parameters in URL were correct.
Suspecting that the problem might be that the URL was too long, I used POST. Here is how I use POST to get the example chart (complete code, you may want to copy it and run):
$postData = array(
'chl' => 'Hello|World',
'chs' => '250x100',
'cht' => 'p3',
'chd' => 't:60,40',
'chof' => 'validate' // turn on the debugging mode
);
$ch = curl_init('http://chart.googleapis.com/chart');
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, $postData);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_HEADER, 0);
curl_setopt($ch, CURLINFO_HEADER_OUT, 1);
$ret = curl_exec($ch);
$info = curl_getinfo($ch);
echo $info["request_header"]; // check my sent header
echo $ret; // print out received data, expected: garbled text representing a PNG
curl_close($ch);
Then Google chart returned (even for this example chart) a 400 page saying that "The Chart API request contains no valid parameters." Notice the word "valid" is here, in contrast to "the Chart API request contains no parameters" upon receiving no parameters at all. Therefore Google Chart must have received some parameters but just that it cannot recognize them.