PHP (CI) cURL passed multidimensional array does n

I'm having an extrange issue when receiving parameters from a POST cURL request. No matter how I encode it (json, url, rawurl, utf8, base64...) before POSTing it, I am not able to perform any decoding operation through the array elements, via loop. I'm giving you the details.

From the consuming controller, in some other php (Yii) app, I build my request like this:

private function callTheApi($options)
{
    $url = "http://api.call.com/url/api";

    $params = array(    'api_key' => $this->api_key,
                        'domain' => $this->domain,
                        'date' => $options['date'],
                        'keys' => $options['keys'] // This is an array
    );

    // Following some good advice from Daniel Vandersluis here:
    // http://stackoverflow.com/questions/3772096/posting-multidimensional-array-with-php-and-curl
    if (is_array($params['keys'])
    {
        foreach ($params['keys'] as $id => $name)
        {
            $params['keys[' . $id . ']'] = $name;
        }
        unset($params['keys']);
    }

    $ch = curl_init();

    curl_setopt($ch, CURLOPT_URL, $url);
    curl_setopt($ch, CURLOPT_HTTPHEADER, array('Content-type: multipart/form-data; charset=utf-8'));
    curl_setopt($ch, CURLOPT_TIMEOUT, 120);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
    curl_setopt($ch, CURLOPT_POST, TRUE);
    curl_setopt($ch, CURLOPT_POSTFIELDS, $params);
    curl_setopt($ch, CURLOPT_USERAGENT, 
            "Mozilla/5.0 (Windows; U; Windows NT 5.0; en; rv:1.9.0.4) "
            . "Gecko/2009011913 Firefox/3.0.6");

    $output = curl_exec($ch);

    $error = curl_errno($ch);
    $error_text = curl_error($ch);

    curl_close($ch);

    if (!$output || $error != 0)
        die("<br><hr>Problems...<br>"
            . "Line:" . __LINE__ . " dataExtractor.php<br>"
            . "Error: " . $error . " - " . $error_text . "<hr>" 
            . $url . "<hr>");

    sleep(1);

    return json_decode($output, true);
}

And in the api itself, this is the function:

public function api()
{
    $params = $_POST;

    foreach($params as $k=>$v){
        if($k=='domain') $domain = $v;
        if($k=='date') $date = $v;
        if($k=='api_key') $api_key = $v;
        if($k=='keys') $keys = $v;
    }

    echo json_encode($keys);
    // All my logic would be here, after parsing the array correctly.
}

Ok, now for the problems:

If i leave everything like stated before, it works. I have my $keys array in the api, and I can use it however I want. The "echo json_encode($keys)" sentence returns the array ALMOST as it should be. But the problem is some values of the array are corrupted in the cURL operation. Values such as spanish characters á, é ,í, ó, ú OR ü are simply not present in the array_values.

If some key in the $keys array was spanish word "alimentación" in the original array, once it's been cURLed to the api, it becomes "alimentacin". There, the ó is not there anymore.

So, my chances are encoding each value in the array to a safely transferred value, so that I can decode it later. But what do you know, I can't.

I've tried urlencoding, rawurlencoding, json_encoding, base64_encoding... each value of the array. And if I return the received array from the api, it contains the encoded values all right. BUT.

If I loop the array in the api for decoding, and then try to return it, no matter what decoding function I'm applying to its values, the output is ALWAYS "NULL".

I have no clue what I'm doing wrong here. Not even close. So any help would be much appreciated. Thanks in advance, community.

When you create cUrl params array you should know that keys cannot be utf8.

And when you add some parameters in foreach loop

$params['keys[' . $id . ']'] = $name;

$id can be utf8 character.

To avoid that I recommend you to use json_encode

$params = array(    
    'api_key' => $this->api_key,
    'domain' => $this->domain,
    'date' => $options['date'],
    'keys' => json_encode($options['keys']) // This is an array
);

In your api in this case you should change nothing.