Detailed explanation of PHP constants: the difference between define and const

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A constant is a simple identifier symbol. This value cannot be changed during script execution (except for so-called magic constants, which are not constants). Constants are case-sensitive by default. Normally constant identifiers are always uppercase.

You can use the define() function to define constants. After php5.3.0, you can use the const keyword to define constants outside the class definition. In previous versions, the const keyword can only be used in classes. Once a constant is defined, it cannot be changed or undefined.

Constant 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.

You can get the value of a constant simply by specifying its name. Unlike variables, you should not add the $ sign in front of a constant. If the constant name is dynamic, you can also use the function constant() to get the value of the constant. Use get_defined_contstants() to get a list of all defined constants.

The differences between constants and variables are as follows:

1, There is no dollar sign ($) in front of the constant

2, Constants can only be defined through the define() function, not through assignment statements

3, Constants can be defined and accessed anywhere regardless of the scope of the variable

4.Once a constant is defined, it cannot be redefined or undefined

5, The value of a constant can only be a scalar

<?php
//
 以下代码在 PHP 5.3.0 后可以正常工作
const USERNAME = &#39;zhouguowei&#39;;
echo USERNAME;
echo constant
("USERNAME");
 
const ZHOUUSERNAME = &#39;zhouguowei2222222222&#39;;
 
define (&#39;MYUSERNAME&#39;,&#39;zhouguowei1111111&#39;);
echo "<pre class="brush:php;toolbar:false">";
print_r( get_defined_constants());  
?>

Q: When defining constants in php, what is the difference between const and define?

Answer: Using 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 during compilation.

1. const is used for the definition of class member variables. Once defined, it cannot be modified. Define cannot be used to define class member variables, but can be used for global constants.

2, Const can be used in a class, define cannot

3, Const Constants cannot be defined in conditional statements

<?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

<?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 scalars, while define can use any expression

<?php
constBIT_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 Sensitive, and define can specify whether it is case sensitive through the third parameter (true indicates case insensitivity).

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

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