有时候我们需要跟安全的 https 网站交互，获取对方内容，如果使用 curl 需要设置额外的选项。

下面给出php示例代码：

其中 cacert.pem 根CA证书文件可以去 http://curl.haxx.se/docs/caextract.html 下载。

$url = 'https://github.com/';  

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $url);

// Set so curl_exec returns the result instead of outputting it.
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);

//verify ssl server
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, true);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 2);

//set CA Certs file path
curl_setopt($ch, CURLOPT_CAINFO, '/home/ssl/cacert.pem');

// Get the response and close the channel.
$response = curl_exec($ch);
curl_close($ch);

return $response;

$curl -h
    Usage: curl [options...] <url>
    Options: (H) means HTTP/HTTPS only, (F) means FTP only
         --anyauth       Pick "any" authentication method (H)
     -a, --append        Append to target file when uploading (F/SFTP)
         --basic         Use HTTP Basic Authentication (H)
         --cacert FILE   CA certificate to verify peer against (SSL)
         --capath DIR    CA directory to verify peer against (SSL)
     -E, --cert CERT[:PASSWD] Client certificate file and password (SSL)
         --cert-type TYPE Certificate file type (DER/PEM/ENG) (SSL)
         --ciphers LIST  SSL ciphers to use (SSL)
         --compressed    Request compressed response (using deflate or gzip)
     -K, --config FILE   Specify which config file to read
         --connect-timeout SECONDS  Maximum time allowed for connection
     -C, --continue-at OFFSET  Resumed transfer offset
     -b, --cookie STRING/FILE  String or file to read cookies from (H)
     -c, --cookie-jar FILE  Write cookies to this file after operation (H)
         --create-dirs   Create necessary local directory hierarchy
         --crlf          Convert LF to CRLF in upload
         --crlfile FILE  Get a CRL list in PEM format from the given file
     -d, --data DATA     HTTP POST data (H)
         --data-ascii DATA  HTTP POST ASCII data (H)
         --data-binary DATA  HTTP POST binary data (H)
         --data-urlencode DATA  HTTP POST data url encoded (H)
         --delegation STRING GSS-API delegation permission
         --digest        Use HTTP Digest Authentication (H)
         --disable-eprt  Inhibit using EPRT or LPRT (F)
         --disable-epsv  Inhibit using EPSV (F)
         --dns-servers    DNS server addrs to use: 1.1.1.1;2.2.2.2
         --dns-interface  Interface to use for DNS requests
         --dns-ipv4-addr  IPv4 address to use for DNS requests, dot notation
         --dns-ipv6-addr  IPv6 address to use for DNS requests, dot notation
     -D, --dump-header FILE  Write the headers to this file
         --egd-file FILE  EGD socket path for random data (SSL)
         --engine ENGINE  Crypto engine (SSL). "--engine list" for list
     -f, --fail          Fail silently (no output at all) on HTTP errors (H)
     -F, --form CONTENT  Specify HTTP multipart POST data (H)
         --form-string STRING  Specify HTTP multipart POST data (H)
         --ftp-account DATA  Account data string (F)
         --ftp-alternative-to-user COMMAND  String to replace "USER [name]" (F)
         --ftp-create-dirs  Create the remote dirs if not present (F)
         --ftp-method [MULTICWD/NOCWD/SINGLECWD] Control CWD usage (F)
         --ftp-pasv      Use PASV/EPSV instead of PORT (F)
     -P, --ftp-port ADR  Use PORT with given address instead of PASV (F)
         --ftp-skip-pasv-ip Skip the IP address for PASV (F)
         --ftp-pret      Send PRET before PASV (for drftpd) (F)
         --ftp-ssl-ccc   Send CCC after authenticating (F)
         --ftp-ssl-ccc-mode ACTIVE/PASSIVE  Set CCC mode (F)
         --ftp-ssl-control Require SSL/TLS for ftp login, clear for transfer (F)
     -G, --get           Send the -d data with a HTTP GET (H)
     -g, --globoff       Disable URL sequences and ranges using {} and []
     -H, --header LINE   Custom header to pass to server (H)
     -I, --head          Show document info only
     -h, --help          This help text
         --hostpubmd5 MD5  Hex encoded MD5 string of the host public key. (SSH)
     -0, --http1.0       Use HTTP 1.0 (H)
         --http1.1       Use HTTP 1.1 (H)
         --http2.0       Use HTTP 2.0 (H)
         --ignore-content-length  Ignore the HTTP Content-Length header
     -i, --include       Include protocol headers in the output (H/F)
     -k, --insecure      Allow connections to SSL sites without certs (H)
         --interface INTERFACE  Specify network interface/address to use
     -4, --ipv4          Resolve name to IPv4 address
     -6, --ipv6          Resolve name to IPv6 address
     -j, --junk-session-cookies Ignore session cookies read from file (H)
         --keepalive-time SECONDS  Interval between keepalive probes
         --key KEY       Private key file name (SSL/SSH)
         --key-type TYPE Private key file type (DER/PEM/ENG) (SSL)
         --krb LEVEL     Enable Kerberos with specified security level (F)
         --libcurl FILE  Dump libcurl equivalent code of this command line
         --limit-rate RATE  Limit transfer speed to this rate
     -l, --list-only     List only mode (F/POP3)
         --local-port RANGE  Force use of these local port numbers
     -L, --location      Follow redirects (H)
         --location-trusted like --location and send auth to other hosts (H)
     -M, --manual        Display the full manual
         --mail-from FROM  Mail from this address (SMTP)
         --mail-rcpt TO  Mail to this/these addresses (SMTP)
         --mail-auth AUTH  Originator address of the original email (SMTP)
         --max-filesize BYTES  Maximum file size to download (H/F)
         --max-redirs NUM  Maximum number of redirects allowed (H)
     -m, --max-time SECONDS  Maximum time allowed for the transfer
         --metalink      Process given URLs as metalink XML file
         --negotiate     Use HTTP Negotiate Authentication (H)
     -n, --netrc         Must read .netrc for user name and password
         --netrc-optional Use either .netrc or URL; overrides -n
         --netrc-file FILE  Set up the netrc filename to use
     -N, --no-buffer     Disable buffering of the output stream
         --no-keepalive  Disable keepalive use on the connection
         --no-sessionid  Disable SSL session-ID reusing (SSL)
         --noproxy       List of hosts which do not use proxy
         --ntlm          Use HTTP NTLM authentication (H)
         --oauth2-bearer TOKEN  OAuth 2 Bearer Token (IMAP, POP3, SMTP)
     -o, --output FILE   Write output to <file> instead of stdout
         --pass PASS     Pass phrase for the private key (SSL/SSH)
         --post301       Do not switch to GET after following a 301 redirect (H)
         --post302       Do not switch to GET after following a 302 redirect (H)
         --post303       Do not switch to GET after following a 303 redirect (H)
     -#, --progress-bar  Display transfer progress as a progress bar
         --proto PROTOCOLS  Enable/disable specified protocols
         --proto-redir PROTOCOLS  Enable/disable specified protocols on redirect
     -x, --proxy [PROTOCOL://]HOST[:PORT] Use proxy on given port
         --proxy-anyauth Pick "any" proxy authentication method (H)
         --proxy-basic   Use Basic authentication on the proxy (H)
         --proxy-digest  Use Digest authentication on the proxy (H)
         --proxy-negotiate Use Negotiate authentication on the proxy (H)
         --proxy-ntlm    Use NTLM authentication on the proxy (H)
     -U, --proxy-user USER[:PASSWORD]  Proxy user and password
         --proxy1.0 HOST[:PORT]  Use HTTP/1.0 proxy on given port
     -p, --proxytunnel   Operate through a HTTP proxy tunnel (using CONNECT)
         --pubkey KEY    Public key file name (SSH)
     -Q, --quote CMD     Send command(s) to server before transfer (F/SFTP)
         --random-file FILE  File for reading random data from (SSL)
     -r, --range RANGE   Retrieve only the bytes within a range
         --raw           Do HTTP "raw", without any transfer decoding (H)
     -e, --referer       Referer URL (H)
     -J, --remote-header-name Use the header-provided filename (H)
     -O, --remote-name   Write output to a file named as the remote file
         --remote-name-all Use the remote file name for all URLs
     -R, --remote-time   Set the remote file's time on the local output
     -X, --request COMMAND  Specify request command to use
         --resolve HOST:PORT:ADDRESS  Force resolve of HOST:PORT to ADDRESS
         --retry NUM   Retry request NUM times if transient problems occur
         --retry-delay SECONDS When retrying, wait this many seconds between each
         --retry-max-time SECONDS  Retry only within this period
         --sasl-ir       Enable initial response in SASL authentication
     -S, --show-error    Show error. With -s, make curl show errors when they occur
     -s, --silent        Silent mode. Don't output anything
         --socks4 HOST[:PORT]  SOCKS4 proxy on given host + port
         --socks4a HOST[:PORT]  SOCKS4a proxy on given host + port
         --socks5 HOST[:PORT]  SOCKS5 proxy on given host + port
         --socks5-hostname HOST[:PORT] SOCKS5 proxy, pass host name to proxy
         --socks5-gssapi-service NAME  SOCKS5 proxy service name for gssapi
         --socks5-gssapi-nec  Compatibility with NEC SOCKS5 server
     -Y, --speed-limit RATE  Stop transfers below speed-limit for 'speed-time' secs
     -y, --speed-time SECONDS  Time for trig speed-limit abort. Defaults to 30
         --ssl           Try SSL/TLS (FTP, IMAP, POP3, SMTP)
         --ssl-reqd      Require SSL/TLS (FTP, IMAP, POP3, SMTP)
     -2, --sslv2         Use SSLv2 (SSL)
     -3, --sslv3         Use SSLv3 (SSL)
         --ssl-allow-beast Allow security flaw to improve interop (SSL)
         --stderr FILE   Where to redirect stderr. - means stdout
         --tcp-nodelay   Use the TCP_NODELAY option
     -t, --telnet-option OPT=VAL  Set telnet option
         --tftp-blksize VALUE  Set TFTP BLKSIZE option (must be >512)
     -z, --time-cond TIME  Transfer based on a time condition
     -1, --tlsv1         Use TLSv1 (SSL)
         --trace FILE    Write a debug trace to the given file
         --trace-ascii FILE  Like --trace but without the hex output
         --trace-time    Add time stamps to trace/verbose output
         --tr-encoding   Request compressed transfer encoding (H)
     -T, --upload-file FILE  Transfer FILE to destination
         --url URL       URL to work with
     -B, --use-ascii     Use ASCII/text transfer
     -u, --user USER[:PASSWORD][;OPTIONS]  Server user, password and login options
         --tlsuser USER  TLS username
         --tlspassword STRING TLS password
         --tlsauthtype STRING  TLS authentication type (default SRP)
     -A, --user-agent STRING  User-Agent to send to server (H)
     -v, --verbose       Make the operation more talkative
     -V, --version       Show version number and quit
     -w, --write-out FORMAT  What to output after completion
         --xattr        Store metadata in extended file attributes
     -q                 If used as the first parameter disables .curlrc