除了field以外，还可以有局部变量(local var)，全局变量（global var，C#不支持），静态变量（static var）。函数的参数也可看成是一种变量。
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Just declaring a global variable without using it in a function also slows things down (by about the same amount as incrementing a local var).
仅定义一个局部变量而没在函数中调用它，同样会减慢速度(其程度相当于递增一个局部变量)。
Also as the name says, implicitly typed local variable, you cannot use var for field declaration or constant variables, therefore following declarations will both produce a compiler error.
故名思意，“隐式类型局部变量”将无法用于域变量或常量的声明中，否则将导致编译错误。
Just declaring a global variable without using it in a function also slows things down (by about the same amount as incrementing a local var). PHP probably does a check to see if the global exists.
仅定义一个局部变量而没在函数中调用它，同样会减慢速度(其程度相当于递增一个局部变量)。PHP大概会检查看是否存在全局变量。
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