关于PHP语言构造器介绍_php技巧
PHP里有echo、print、die、require等几个特殊的关键字,虽然它们用起来像是函数,但实际上更类似于if、while这样控制语句,而不是一个函数。也就是说,当解释器遇到:
print 'Hello world';
这样的一个表达式的时候,并不会把它转换成函数调用,而是直接映射到一系列预先定义好的操作。使用语言构成的时候可以加括号,也可以不加括号,但是使用函数的时候必须加括号。
你提到的“可变函数”,英文是”variable function”,变量函数的意思,翻译成“可变函数”也有点扭曲原义,把variable(变量)这个名词按形容词(可变)的含义来翻译了。PHP里的Variable function是指如果在一个变量后面加上一对括号,那么解释器会试图寻找名字和这个变量的值一样的一个函数,如果找到了就执行它。例如,有一个函数foo(),那么可以用下面这种方式调用这个函数:
// 初始化一个字符串变量
$func = 'foo';
// 找到名字和这个字符串一样的函数,并且执行它
$func();
所以,你提到的这句话的含义就是,使用变量函数这种方式来调用一个语言构成是不允许的,例如下面这样:
$func = 'print';
// 这样做会产生异常,因为print不是一个函数,而是语言的构成部分
$func('hello world');
执行这段代码,会产生一个异常,函数print没有定义。

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