curl_init()
返回一个 curl handle
curl_setopt()
为curl handle设置参数，格式为 curlopt_xxx
常用参数：
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curlopt_url：url
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curlopt_referer：referer提交的页面
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curlopt_post：是否是post请求
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curlopt_timeout：超时时间
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curlopt_httpheader：head请求数据
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curlopt_header：
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curlopt_ssl_verifypeer：false to stop curl from verifying the peer’s certificate. alternate certificates to verify against can be specified with the curlopt_cainfo option or a certificate directory can be specified with the curlopt_capath option.
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curlopt_ssl_verifyhost：1 to check the existence of a common name in the ssl peer certificate. 2 to check the existence of a common name and also verify that it matches the hostname provided. 0 to not check the names. in production environments the value of this option should be kept at 2 (default value).
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curlopt_returntransfer：true to return the transfer as a string of the return value of curl_exec() instead of outputting it directly.
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curlopt_postfields：post提交数据
curl_exec()
执行请求，如果设置了 curlopt_returntransfer，成功会返回结果，失败返回false。
curl_close()
关闭
