how to set custom HTTP header in CURL with php ?

$url='your web api url';
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_HTTPHEADER,array(User-Agent: PHP-SOAP-CURL ) );
$response = curl_exec($ch);
curl_close($ch);	

$opts = array('http' =>
    array(
        'method'  => 'POST',
        'header'  => 'Content-type: application/x-www-form-urlencoded;User-Agent: PHP-SOAP-CURL'
    )
);

$context = stream_context_create($opts);

$result = file_get_contents('http://example.com', false, $context);

$host="127.0.0.1";
$ch = curl_init();
$headers = array(
 	'Connection: keep-alive',
	'User-Agent: PHP-SOAP-CURL',
	'Content-Type: application/json; charset=utf-8',
	'Accept: application/json',
	'Content-Length: ' . strlen($data_string),
	'Host '.$host);
curl_setopt($ch, CURLOPT_URL, $this->url);
curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
$response = curl_exec($ch);
curl_close($ch);	
print_r($response);