Detailed analysis of php constants, php constants_PHP tutorial

Detailed analysis of php constants, php constants

1. Constants

Constant is an identifier (name) of a simple value. As the name implies, the value cannot change during script execution (except for so-called magic constants, which are not constants). Constants are case-sensitive by default. By convention constant identifiers are always uppercase.
Constant names follow the same naming rules as any other PHP tag. Legal constant names begin with a letter or an underscore, followed by any letters, numbers, or underscores. The regular expression using is expressed as follows :
[a-zA-Z_x7f-xff][a-zA-Z0-9_x7f-xff]* (Note: Here, the letters are a-z, A-Z, and from 127 to 255 (ASCII characters 0x7f-0xff)
Like super globals, the scope of constants is global. Constants can be accessed anywhere in the script regardless of scope. For more information about functions please read the Variable Scope manual.

2. Grammar
Constants can be defined using the define() function. Once a constant is defined, it cannot be changed or undefined during program execution.
Constants can only contain scalar data (boolean, integer, float and string).
You can obtain the value of a constant simply by specifying its name. Do not precede the constant with the $ sign. If the constant name is dynamic, you can also use the function constant() to read the value of the constant. Use get_defined_constants() to get a list of all defined constants.
Note: Constants and (global) variables are in different namespaces. This means for example TRUE and $TRUE are different.
If an undefined constant is used, PHP assumes that you want the name of the constant itself, as if you were calling it as a string (CONSTANT corresponds to "CONSTANT"). An E_NOTICE level error will be issued. See the manual for why $foo[bar] is wrong (unless you define bar as a constant with define() beforehand). If you just want to check whether a certain constant is defined, use the defined() function.

3. The difference between constants and variables
There is no dollar sign ($) in front of the constant;
Constants can only be defined using the define() function, not assignment statements;
Constants can be defined and accessed anywhere regardless of variable scope rules;
Once defined, a constant cannot be redefined or undefined;
The value of a constant can only be a scalar value.
Example of constant definition:

<&#63;php 
define("CONSTANT", "Hello world."); 
echo CONSTANT; // outputs "Hello world." 
echo Constant; // outputs "Constant" and issues a notice. 
&#63;> 

4. Predefined constants
PHP provides a large number of predefined constants to any script it runs. However, many constants are defined by different extension libraries and will only appear when these extension libraries are loaded, either dynamically loaded or included at compile time.
There are four magic constants that change depending on where they are used. For example, the value of __LINE__ depends on the line it is in the script. These special constants are not case-sensitive, as follows:
Several PHP “magic constants”
__LINE__ The current line number in the file.
__FILE__ The full path and file name of the file.
__FUNCTION__ function name (newly added in PHP 4.3.0).
__CLASS__ The name of the class (new in PHP 4.3.0).
__METHOD__ The method name of the class (newly added in PHP 5.0.0).

The above is a detailed introduction to the basic content of PHP constants. I hope it will be helpful to everyone's learning.

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