PHP basic constants

Constant

1. Constant composition = constant name + constant value

Constant name: [a- zA-Z0-9_] combination , and numbers cannot be used as the beginning, case-sensitive, uppercase is recommended

Constant value: assignable data type Integer, floating point, Boolean, string, null

2. How to declare constants

define( 'Constant name','Constant value')

const Constant name = constant value (supported after PHP 5.3)

3. Detection of constants bool defined (constant name)

4. Characteristics of constants:

(1) Once defined, it cannot be redeclared

( 2) Once defined, it cannot be reassigned

5. Variable constant constant (constant name)

6. The difference between define and const

(1) "Position" can be declared

define: line of code, control structure, loop structure, function

const: line of code, class

Note:

Line of code: not in the control structure, loop body, function, class, but a pure line of code

The constants declared with define in the function can only be used after the function is called, otherwise an error will be reported as a character String output

(2) Scope

define and const Constants declared in the code line have global validity

const Constants declared in the class , can only be used in classes

(3) const uses an ordinary constant name, define can use an expression as the name

const FOO = 'BAR';

for ($i = 0; $i < 32; ++$i) {

define('BIT_' . $i, 1 << $i);

}

(4) const can only accept static scalars, while define can take any expression

For example :

_Const Bit_5 = 1 & LT; & LT; 5; // Invple Invalid

Define ('Bit_5', 1 & LT; & LT; 5); // Valid Valid

(5) Constants defined by const are case-sensitive, and define can specify whether case-sensitive is through the third parameter (true means case-insensitive)

For example:

define('FOO', 'BAR', true);

echo FOO; // BAR

echo foo;

(6)

Using const makes the code simple and easy to read. const itself is a language structure, and define is a function

(7)

const is much faster than define during compilationIllustration:

Image resource address: http://download.csdn.net/detail/zz249456649/8571357

Just a personal comment

Definition of constants: When the page is running or in a class, declare a constant value

Usage:

Formulas, project configuration files, website root directories, etc.

The above has introduced the basic constants of PHP, including related content. I hope it will be helpful to friends who are interested in PHP tutorials.