一个可扩展的PHPWEB蜘蛛，示例代码：
use VDB\Spider\Spider;use VDB\Spider\Discoverer\XPathExpressionDiscoverer;$spider = new Spider('https://www.oschina.net');特性：
supportstwotraversalalgorithms:breadth-firstanddepth-first
supportsdepthlimitingandqueuesizelimiting
supportsaddingcustomURIdiscoverylogic,basedonXPath,CSSselectors,orplainoldPHP
comeswithausefulsetofURIfilters,suchasDomainlimiting
supportscustomURIfilters,bothprefetch(URI)andpostfetch(Resourcecontent)
supportscustomrequesthandlinglogic
comeswithausefulsetofpersistencehandlers(memory,file.Redissoontofollow)
supportscustompersistencehandlers
collectsstatisticsaboutthecrawlforreporting
dispatchesusefulevents,allowingdeveloperstoaddevenmorecustombehavior
supportsapolitenesspolicy
willsooncomewithmanydefaultdiscoverers:RSS,Atom,RDF,etc.
willsoonsupportmultiplequeueingmechanisms(file,memcache,redis)
willeventuallysupportdistributedspideringwithacentralqueue
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