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.

在一个函数里如果只声明一个全局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很有可能会检查看是否存在全局变量。

youdao