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42 ” cURL Extension containing the authentication credentials to use in the format ’username:password’. N ote that this has to be set for each request requiring authentication. R edir ection CURLOPT_FOLLOWLOCATION can be set to true to have cURL automatically place process redirections. That is, it will detect Location headers in the server response and implicitly issue requests until the server response no longer contains a Location header. To set the maximum number of Location headers to have cURL process automatically before terminating, use the CURLOPT_MAXREDIRS setting. To have authentication credentials persist in requests resulting from redirections, set the CURLOPT_UNRESTRICTED_AUTH setting to true. R efer ers CURLOPT_REFERER allo ws you to explicitly set the value of the Referer header. Setting CURLOPT_AUTOREFERER to true will cause cURL to automatically set the value of the Referer header whenever it processes a Location header. Content Caching CURLOPT_TIMECONDITION must be set to either CURL_TIMECOND_IFMODSINCE or CURL_TIMECOND_IFUNMODSINCE to select whether the If-Modified-Since or If-Unmodified-Since header will be used respectively. CURLOPT_TIMEVALUE must be set to a UNIX timestamp (a date representation using the number of seconds between the UNIX epoch and the desired date) to indicate the last client access time of the resource. The time function can be used to derive this value. U ser Agents CURLOPT_USERAGENT can be used to set the U ser Agent string to use.
cURL Extension ” 43 B yte Ranges CURLOPT_RESUME_FROM can be used to set a single point within the document from which to start the response body. This will cause the Range header to be set with a value of X- where X is the specified starting point. In order to specify multiple ranges, CURLOPT_RANGE accepts a string in the same format as the Range header (ex: X-Y,Z-). DNS Caching You may notice that code using the cURL extension appears to run faster than code using streams. The reason for this is that cURL implements its o wn DNS cache, which is more likely to be apparent if your operating system or internet service pro vider does not pro vide one. DNS, or Domain N ame System, is a system used to derive an IP address for a domain name in a manner similar to ho w phone directories are used to obtain a phone number for a person using their name. The process of obtaining an IP address for a domain name, called a DNS lookup , can be a costly operation in terms of the time required. Because the results of DNS lookups don’t change often, DNS caching is often used to retain the results of lookups for a certain time period after they are performed. This can be done at multiple levels including the source code level as with cURL, natively at the OS level, or via software like nscd or dnsmasq run either locally or on remote servers such as those used by internet service pro viders. cURL DNS caching is enabled by default. Some situations like debugging may warrant disabling it, which can be done by setting CURLOPT_DNS_USE_GLOBAL_CACHE to false. cURL will also by default retain the results of DNS lookups in memory for two minutes. To change this, set the CURLOPT_DNS_CACHE_TIMEOUT setting to the number of seconds a result should remain in the cache before expiring. Also noteworthy is the fact that cURL DNS caching is not thread-safe. Threading is a particular style of parallel processing. The most common implementation of threading consists of multiple threads of execution contained within a single operating system process that share resources such as memory. Because of this, it may
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vi ” CONTENTS Referring URLs . .
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viii ” CONTENTS HTTP Authenticati
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Chapter 13 CSS Selector Libraries T
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PCRE Extension ” 145 Anchors Y o
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PCRE Extension ” 147 // Matches
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PCRE Extension ” 149 if (preg_mat
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A p p e n d i x A Legality of W e S
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