PHP 使用 curl 类库，获取请求的回应头（Response Headers），下面总结了2种方法。

1. 使用 CURLOPT_HEADER 选项

设置选项 curl_setopt() 方法，当 CURLOPT_HEADER 设置 true ，curl_exec 结果将输出回应头。这时如果 CURLOPT_NOBODY 设置为 false ，curl_exec 将返回回应头和内容 body，否则只返回回应头。

curl_getinfo($ch, CURLINFO_HEADER_SIZE) 返回回应头的字符长度，方便将 header 和 body 区分出来。

$url = "https://www.baidu.com";
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_HEADER, 1);
// 1 表示只返回请求头
curl_setopt($ch, CURLOPT_NOBODY, 0);
$response = curl_exec($ch);

// curl_exec 之后
$header_size = curl_getinfo($ch, CURLINFO_HEADER_SIZE);
$header = substr($response, 0, $header_size);
$body = substr($response, $header_size);
echo $header;
// echo $body;

输出

HTTP/1.1 200 OK
Accept-Ranges: bytes
Cache-Control: no-cache
Connection: keep-alive
Content-Length: 14722
Content-Type: text/html
Date: Thu, 23 Apr 2020 15:02:52 GMT
P3p: CP=" OTI DSP COR IVA OUR IND COM "
P3p: CP=" OTI DSP COR IVA OUR IND COM "
Pragma: no-cache
Server: BWS/1.1
Set-Cookie: BAIDUID=2075A60690A5E152C301511B9CA6982A:FG=1; expires=Thu, 31-Dec-37 23:55:55 GMT; max-age=2147483647; path=/; domain=.baidu.com
Set-Cookie: BIDUPSID=2075A60690A5E152C301511B9CA6982A; expires=Thu, 31-Dec-37 23:55:55 GMT; max-age=2147483647; path=/; domain=.baidu.com
Set-Cookie: PSTM=1587654172; expires=Thu, 31-Dec-37 23:55:55 GMT; max-age=2147483647; path=/; domain=.baidu.com
Set-Cookie: BAIDUID=2075A60690A5E1527030AB064F519CA8:FG=1; max-age=31536000; expires=Fri, 23-Apr-21 15:02:52 GMT; domain=.baidu.com; path=/; version=1; comment=bd
Traceid: 1587654172022180301811607925382683554957
Vary: Accept-Encoding
X-Ua-Compatible: IE=Edge,chrome=1

2. 使用 CURLOPT_HEADERFUNCTION 选项

CURLOPT_HEADERFUNCTION 选项，设置一个回调函数，这个函数有两个参数，第一个是cURL的资源句柄，第二个是输出的 header 数据。header数据的输出必须依赖这个函数，返回已写入的数据大小。

$headers = [];
$url = "https://www.baidu.com";
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_HEADERFUNCTION,
    function ($curl, $header) use (&$headers) {
        $len = strlen($header);
        $header = explode(':', $header, 2);
        if (count($header) < 2) // ignore invalid headers
            return $len;

        $headers[strtolower(trim($header[0]))][] = trim($header[1]);

        return $len;
    }
);
$response = curl_exec($ch);
print_r($headers);

将输出

Array
(
    [accept-ranges] => Array
        (
            [0] => bytes
        )

    [cache-control] => Array
        (
            [0] => no-cache
        )

    [connection] => Array
        (
            [0] => keep-alive
        )

    [content-length] => Array
        (
            [0] => 14722
        )

    [content-type] => Array
        (
            [0] => text/html
        )

    [date] => Array
        (
            [0] => Thu, 23 Apr 2020 15:01:42 GMT
        )

    [p3p] => Array
        (
            [0] => CP=" OTI DSP COR IVA OUR IND COM "
            [1] => CP=" OTI DSP COR IVA OUR IND COM "
        )

    [pragma] => Array
        (
            [0] => no-cache
        )

    [server] => Array
        (
            [0] => BWS/1.1
        )

    [set-cookie] => Array
        (
            [0] => BAIDUID=C0D84316D0C85F1E22A6685A73ADE4BE:FG=1; expires=Thu, 31-Dec-37 23:55:55 GMT; max-age=2147483647; path=/; domain=.baidu.com
            [1] => BIDUPSID=C0D84316D0C85F1E22A6685A73ADE4BE; expires=Thu, 31-Dec-37 23:55:55 GMT; max-age=2147483647; path=/; domain=.baidu.com
            [2] => PSTM=1587654102; expires=Thu, 31-Dec-37 23:55:55 GMT; max-age=2147483647; path=/; domain=.baidu.com
            [3] => BAIDUID=C0D84316D0C85F1EF148BCBDC9AEE4EB:FG=1; max-age=31536000; expires=Fri, 23-Apr-21 15:01:42 GMT; domain=.baidu.com; path=/; version=1; comment=bd
        )

    [traceid] => Array
        (
            [0] => 1587654102022199962611480622685312756450
        )

    [vary] => Array
        (
            [0] => Accept-Encoding
        )

    [x-ua-compatible] => Array
        (
            [0] => IE=Edge,chrome=1
        )

)