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.

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>

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

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

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See the scope of constants clearly

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

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