在代码部分，定义了一个函数地址，方法同上述定义全局变量的相同——只需在它们的名称之后跟上一个冒号。
In the code section, you define a function's address the same way you defined addresses for global variables above — just put their name followed by a colon.
函数可在另一个函数内定义，还可赋给全局变量或作为结果返回。
Functions can be defined within another function and assigned to global variables or returned as results.
一个扩展可以定义标签、过滤器、测试、操作、全局变量、函数和节点访问等。
An extension can define tags, filters, tests, operators, global variables, functions, and node visitors.
exists函数确定是否定义了一个具有由字符串定义的名称的变量(在本例中为全局变量g： syntax_on)。
The exists function determines whether a variable with the name specified by the string (in this case, the global variable g: syntax_on) has been defined.
在函数内定义一个与函数可能会用到的全局变量同名的局部变量总是不好的。
It is almost always a bad idea to define a local variable with the same name as a global variable that the function USES or might use.
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。
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。
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.
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