curl CommandLine Uniform Resource Locator
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