Constants in PHP

Overview

Constant values ​​cannot change during script execution

Once defined, constants cannot be redefined or undefined

Constants are case-sensitive by default. Traditionally constant identifiers are always uppercase. Use the define() function to define constants. After PHP 5.3.0, you can use the const keyword to define constants outside the class definition.

You cannot use const to define constants within functions, loops, and if statements.

Constants can only contain scalar data (boolean, integer, float and string). It is possible to define resource constants, but this should be avoided as it can cause unpredictable results.

Unlike variables, you cannot put the $ sign in front of constants

<code><?php define("$a", "something");
echo $a; // 无效
?>
</code>

Scope of constants

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Constants can be defined and accessed anywhere regardless of the scope of variables

<code><?php define("A", "something");
function fn() {
    echo A;
}
fn(); // something
?>

<?php define("A", "something");
echo A; // something
?>
</code>

See the scope of constants clearly

<code><?php echo A; // A
define("A", "something");
?>
</code>

Magic constants

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The so-called magic constants are actually not constants

There are eight magic constants, and their values ​​change as their positions in the code change

Name__LINE____FILE____DIR____FUNCTION____CLASS____TRAIT____METHOD____NAMESPACE__

Description
The current line number in the file.
The full path and file name of the file. If used within an included file, returns the name of the included file. Since PHP 4.0.2, __FILE__ always contains an absolute path (or the resolved absolute path in the case of a symbolic link), while versions before that sometimes contained a relative path.
The directory where the file is located. If used within an included file, returns the directory where the included file is located. It is equivalent to dirname(__FILE__). Directory names do not include the trailing slash unless they are the root directory. (New in PHP 5.3.0)
Function name (New in PHP 4.3.0). Since PHP 5 this constant returns the name of the function as it was defined (case sensitive). In PHP 4 this value is always lowercase.
The name of the class (new in PHP 4.3.0). Since PHP 5 this constant returns the name of the class when it was defined (case sensitive). In PHP 4 this value is always lowercase. The class name includes the scope in which it is declared (e.g. FooBar). Note that since PHP 5.4 __CLASS__ also works for traits. When used within a trait method, __CLASS__ is the name of the class that calls the trait method.
Trait name (new in PHP 5.4.0). Since PHP 5.4 this constant returns the name of the trait as it was defined (case-sensitive). The trait name includes the scope in which it is declared (e.g. FooBar).
The method name of the class (newly added in PHP 5.0.0). Returns the name of the method when it was defined (case-sensitive).
The name of the current namespace (case sensitive). This constant is defined at compile time (new in PHP 5.3.0).

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