HTTPS is secure HTTP communication based on SSL protocol (HTTP over SSL). Generally all sensitive info (like passwords, financial details, etc.) are sent over this transport. Common example: your gmail login is done through HTTPS channel and different payment gateway.
So here in this deal –

$postfields = array('field1'=>'value1', 'field2'=>'value2');
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, 'https://foo.com');
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_FOLLOWLOCATION, true);
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, $postFields);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 0);
$result = curl_exec($ch);

CURLOPT_SSL_VERIFYHOST is off. This allows you to test the CURL in your dev server without having HTTPS. PHP/Curl will handle the http request.