The difference and usage examples of PHP constants const and define

The so-called constant is a simple identifier. This value cannot be changed during script execution. Constants are case-sensitive by default. Normally constant identifiers are always uppercase. Constants can only contain scalar data (boolean, integer, float, and string). Resource constants can be defined, but should be avoided as they can cause unpredictable results.

define part
Macros can be used not only to replace constant values, but also to replace expressions and even code segments. (The function of macros is very powerful, but it is also error-prone, so its pros and cons are quite controversial.)
The syntax of macros is:

#define 宏名称 宏值

As a suggestion and a common practice among programmers As a general rule, macro names often use all uppercase letters.
Advantages of using macros:
1) Make the code more concise and clear
Of course, this depends on you giving the macro an appropriate name. Generally speaking, macro names should have clear and intuitive meanings, and sometimes it is better to make them longer.
2) Facilitate code maintenance
The processing of macros is called "preprocessing" during the compilation process. That is to say, before formal compilation, the compiler must first replace the macros that appear in the code with their corresponding macro values. This process is similar to the search and replace you and I use in word processing software. Therefore, when macros are used to express constants in the code, in the final analysis, immediate numbers are used, and the type of this quantity is not clearly specified.
const part
The format of constant definition is:

const 数据类型 常量名 = 常量值;

The constant defined by const has a data type, and the constant defining the data type facilitates the compiler to check the data. , so that the program may have errors for troubleshooting. A constant must initially specify a value, and then, in subsequent code, we are not allowed to change the value of this constant.

The difference between const and define:
In the allocation of memory space. When define defines a macro, it will not allocate memory space. It will be replaced in the main function during compilation. It is just a simple replacement without any checks, such as type, statement structure, etc. That is, the macro definition constant is just a pure placement. Relationship, such as #define null 0; the compiler always replaces null with 0 when it encounters null. It has no data type (if you have any questions, please look at the preprocessing part of the C language book or MSDN. The constants defined by const have data types. , defining constants of data types facilitates the compiler to check data and troubleshoot possible errors in the program. Therefore, the difference between const and define is that defining constants with const eliminates the insecurity between programs.

Use const makes the code simple and easy to read. Const itself is a language structure, and define is a function. In addition, const is much faster than define at compile time.

1. Const is used for the definition of class member variables. , cannot be modified. Define cannot be used for the definition of class member variables, but can be used for global constants

2. Const can be used in classes, define cannot be used

3. Const cannot be used in conditional statements. Define constants in, the code is as follows:

<?php
if (...){
const FOO = &#39;BAR&#39;;    // 无效的invalid
}
if (...) {
define(&#39;FOO&#39;, &#39;BAR&#39;); // 有效的valid
}
?>

4. const uses ordinary constant names, define can use expressions as names, the code is as follows:

<?php
const  FOO = &#39;BAR&#39;;
for ($i = 0; $i < 32; ++$i) {
define(&#39;BIT_&#39; . $i, 1 << $i);
}
?>

5, const can only accept static scalar, and define can use any expression. The code is as follows:

<?php
const BIT_5 = 1 << 5;    // 无效的invalid
define(&#39;BIT_5&#39;, 1 << 5); // 有效的valid
?>

6. Constants defined by const are case-sensitive, while define can pass the third parameter (true means case-insensitive) To specify whether case is sensitive, the code is as follows:

<?php
define(&#39;FOO&#39;, &#39;BAR&#39;, true);
echo FOO; // BAR
echo foo; // BAR
?>

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