使用PHP列出.7z，.rar和.tar档案中的文件(List files in .7z, .rar and .tar archives using PHP)

 如果你require两次相同的文件，它将被加载和评估一次。 另一方面，每次加载和评估文件。 实际文件名的解析方式也有所不同（ 谢谢，Saurabh ）。 
 
 这实际上意味着什么？ 
 
 假设我们有一个库foo 
 
# foo.rb

class Foo
  def bar
    puts 'bar'
  end

  def quux
    puts 'quux'
  end
end
 
 然后我们有一个文件可以做一些非幂等的操作。 说，没有定义一种方法 
 
# mod.rb
class Foo
  undef :bar
end
 
 那么，如果我们require mod.rb两次，没有什么不好的事情发生。 bar成功未定义。 
 
# main.rb
require './foo'

Foo.instance_methods(false) # => [:bar, :quux]

require './mod'
require './mod'

Foo.instance_methods(false) # => [:quux]
 
 但是如果我们load mod.rb两次，那么第二个undef操作将会失败，因为方法已经消失了： 
 
# main.rb
require './foo'

Foo.instance_methods(false) # => [:bar, :quux]

load 'mod.rb'
load 'mod.rb'

Foo.instance_methods(false) # => 
# ~> mod.rb:2:in `<class:Foo>': undefined method `bar' for class `Foo' (NameError)
# ~>    from mod.rb:1:in `<top (required)>'
# ~>    from -:6:in `load'
# ~>    from -:6:in `<main>'
 
 require没有错误，因为在这种情况下， undef只会发生一次。 诚然，这个例子很有意思，但我希望它能说明这一点。 

If you require the same file twice, it will be loaded and evaluated only once. load, on the other hand, loads and evaluates the file every time. There are also differences in how actual filename is resolved (thanks, Saurabh).
 
What does this mean practically?
 
Let's say we have a library foo
 
# foo.rb

class Foo
  def bar
    puts 'bar'
  end

  def quux
    puts 'quux'
  end
end
 
Then we have a file which makes some non-idempotent operations. Say, undefines a method
 
# mod.rb
class Foo
  undef :bar
end
 
Then, if we require mod.rb twice, nothing bad happens. bar gets successfully undefined.
 
# main.rb
require './foo'

Foo.instance_methods(false) # => [:bar, :quux]

require './mod'
require './mod'

Foo.instance_methods(false) # => [:quux]
 
But if we load mod.rb twice, then second undef operation will fail, because method is already gone:
 
# main.rb
require './foo'

Foo.instance_methods(false) # => [:bar, :quux]

load 'mod.rb'
load 'mod.rb'

Foo.instance_methods(false) # => 
# ~> mod.rb:2:in `<class:Foo>': undefined method `bar' for class `Foo' (NameError)
# ~>    from mod.rb:1:in `<top (required)>'
# ~>    from -:6:in `load'
# ~>    from -:6:in `<main>'
 
There's no error with require because in that case undef happens only once. Granted, this example is quite contrived, but I hope it illustrates the point.