An introduction to the PHP language constructor

PHP contains several special keywords, such as echo, print, die, require, etc. Although they sound like functions, they are actually more like control statements like if and while than a function. That is, when the interpreter encounters:

print 'Hello world';

Such an expression does not translate into function calls, but maps directly to a series of predefined operations. Languages can be constructed with or without parentheses, but functions must be constructed with parentheses.

variable function is a variable function, which means "variable function". It means "variable function". Variable function in PHP means that if a variable is followed by a pair of parentheses, the interpreter tries to find a function with the same name as the variable's value, and if found, executes it. For example, if you have a function called foo(), you can call it this way:

//  Initialize the 1 String variables  
$func = 'foo'; 

//  Find the name and the string 1 Function, and execute it  
$func();

So, what you mean by that is that it is not allowed to use variable functions to call a language structure in this way, for example:

$func = 'print'; 

//  This creates an exception because print not 1 A function, but a building block of language  
$func('hello world');

Executing this code produces an exception, and the function print is not defined.