Playing with php constants_PHP tutorial

Naming rules for constants

Naming: Same naming rules as variables

Method: define() function

Format: define('constant name','specific value');

Example: define('PI',3.14);

Comparison of constants and variables

Same:

1: The naming rules are the same (but generally in uppercase letters)

Different:

1: Don’t use $ when referencing constants, use names directly.

2: Constants can only be defined using define, not assignment statements

3: Once a constant is defined, it cannot be redefined or undefined

4: The value of a constant can only be a scalar (can only be integer, floating point, Boolean, character, NULL)

[php]

//Define constants

define('pi',3.14);

//Calculate the area of ​​the park

$r=3;

echo pi*$r*$r;

//Result: 28.26

//After a constant is defined, it cannot be redefined

define('pi',3.23);

//Result: Notice: Constant pi already defined in C:wampwww

//Constant cannot be reassigned

pi=100;

echo pi;

//Result: Parse error: syntax error, unexpected '=' in C:wampwww

//There will be grammatical errors

$PI=10;

function text(){

echo $PI;//The function has scope. This $PI variable is inside the function and is not the same as the $PI outside.

echo pi;//Constant is defined once and can be used anywhere

}

//Result: Notice: Undefined variable: PI in C:wampwww

text();

?>

How to determine whether a constant is defined?

define defines constants

defined judgment constant

[php]

if(defined('PI')){

echo 'PI constant has been defined.
';

}else{

echo 'PI constant is undefined, let me define it
';

define('PI',33322);

}

echo PI;

?>

[php]

Result: PI constant is undefined, let me define it

33322

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www.bkjia.comtruehttp: //www.bkjia.com/PHPjc/477800.htmlTechArticleNaming rules for constants: The same naming rules as variables Method: define() Function format: define(constant name , specific value); Example: define(PI,3.14); The comparison between constants and variables is the same: 1: Naming...