C Language Structure function: The difference between PHP's language structure and function

I believe you often see comparison of some PHP applications, said with Isset () to replace strlen (), isset faster than strlen execution speed.
Example:
if (Isset ($user)) {
Do some thing
}
The reason is that isset is a language structure, and strlen is a function. And Echo is a language structure, not a function.
What is the language structure? Is it different from a function?
1. What is language structure and function
Language structure: Is the keyword of PHP language, part of language grammar; it cannot be defined by the user or added to a language extension or library; it can have or can have no variables and return values.
Function: Composed of code blocks, can be reused. From the source point of view, that is, based on the basis of the Zend engine to achieve, Ext expansion Library functions are implemented in this way.
2, the language structure is faster than the function reason is in PHP, the function is first by the PHP parser (Zend Engine) decomposed into language structure, so there is this visible, function than the language structure of more than a layer of parser parsing. In this way, we can better understand what language structure is faster than the function.
3, language structure and function of different language structure than the corresponding function of the fast language structure in the error handling is relatively robust, because it is a language keyword, so do not have the process of reprocessing language structure can not be disabled in configuration items (php.ini), the function can be. Language structures cannot be used as callback functions
4, Language structure list
Echo ()
Print ()
Die ()
Isset ()
Unset ()
Include (), note that include_once () is a function
Require (), note that require_once () is a function
Array ()
List ()
Empty () This article links http://www.cxybl.com/html/wlbc/Php/20130326/37403.html