Constants of PHP Basics

Constants

1, the composition of the constant = constant Name + constant value

Constant name: A combination of [a-za-z0-9_], and the number can not be started, case-sensitive, recommended capitalization

Constant value: Assignable data type integral type, float, Boolean, string, NULL

2. How to declare a constant

Define (' Constant name ', ' constant value ')

const constant NAME = constant Value (supported after PHP 5.3)

3, the constant detection bool defined (constant name)

4, the characteristics of the constant:

(1) Once defined, cannot be re-declared

(2) Once defined, the value cannot be re-assigned

5. Variable constant constant (constant name)

6, define and const difference

(1) can declare "location"

Define: Line of code, control structure, loop structure, function

Const: Line of code, class

Note:

Line of code: not control structure, loop body, function, class, Pure line of code

Constants declared with define in a function can only be used after a function call, otherwise an error will be made when the string output

(2) Scope

Constants declared by define and const in code lines, with global validity

Const constants declared in a class can only be used in a class

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

Const FOO = ' BAR ';

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

Define (' Bit_ '. $i, 1 << $i);

}

(4) const can only accept static scalar, and define may take any expression

For example:

Const BIT_5 = 1 << 5; Invalid invalid

Define (' Bit_5 ', 1 << 5); The effective valid

(5) const-defined constants are case-sensitive, while define can specify case sensitivity by using the third argument (true for case insensitivity)

For example:

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

Echo FOO; BAR

echo foo; BAR

(6) using const makes the code easy to read, and const itself is a language structure, and define is a function

(7) const is much faster at compile time than define

Graphic:

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

Personally, talk to yourself.

Definition of a constant: in a page run or class, declare a value that is always constant

use: formulas, project profiles, site roots, and so on

The above describes the basic PHP constants, including the aspects of the content, I hope that the PHP tutorial interested in a friend helpful.

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