使用基本表单发送信息Curl

I'm new using cURL, and I'm trying to send information from an HTTP form through cURL.

If I use $data_string without the form, the code works. How can I pass the values of the form?

<?php

    $data = array('dst_number' => 'dst_number','sms_content' =>     'sms_content');
    $data_string = json_encode($data);
    $ch = curl_init();
    curl_setopt($ch, CURLOPT_URL, 'https://api.connectus.cl/api_v2/send_sms');
    curl_setopt($ch, CURLOPT_CONNECTTIMEOUT, 20);
    curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "POST");
    curl_setopt($ch, CURLOPT_POST, true);
    curl_setopt($ch, CURLOPT_POSTFIELDS, $data_string);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
    curl_setopt($ch, CURLOPT_USERPWD, 'xxxxxxxxxxxxxx:xxxxxxxxxxxxxx');
    curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
    curl_setopt($ch, CURLOPT_HTTPAUTH, CURLAUTH_BASIC);
    curl_setopt($ch, CURLOPT_HEADER, 1);
    curl_setopt($ch, CURLOPT_HTTPHEADER, array(
    'Accept: application/json',
    'Content-Type: application/json')
    );
    if(curl_exec($ch) === false){
    echo 'Curl error: ' . curl_error($ch);
    }
    $response = curl_exec($ch);
    curl_close($ch);
    //print $ch;
    ?>

<html>
    <head>
        <title>SMS</title>
    </head>
    <body>

        <form method="post" action="<?php echo $_SERVER['PHP_SELF']     ?>">
        <p>DESTINATARIO: <input type="text" value="" id="dst_number" name="dst_number" /></p>
        <p>Mensaje: <input type="text" value="" id="sms_content" name="sms_content" maxlength="160" /></p>
        <p><input type="submit" value="Send SMS" name="send" /></p>
        </form>
    </body>
</html>