The difference and connection between PHP predefined constants and user-defined constants

Title: The difference and connection between PHP predefined constants and user-defined constants

In PHP programming, a constant is a fixed value that remains unchanged throughout the script. Can be used in all. Constants come in different types, including predefined constants and user-defined constants. Predefined constants are provided by PHP, and user-defined constants are defined by programmers.

1. PHP predefined constants

Predefined constants are constants defined by the PHP programming language itself and can be used directly in scripts without additional definitions. Common PHP predefined constants include the following:

• FILE: Indicates the file path of the current script.
• LINE: Indicates the current line number.
• DIR: Indicates the directory where the current script is located.
• PHP_VERSION: Indicates the current PHP version number.

The following is a simple code example that demonstrates how to use PHP predefined constants:

echo "Current file path:" . __FILE__ . "<br>";
echo "Current line number:" . __LINE__ . "<br>";
echo "Current directory:" . __DIR__ . "<br>";
echo "Current PHP version number:" . PHP_VERSION . "<br>";

2. User-defined constants

User-defined constants are constants defined by programmers in scripts. Need to use define() function to define. User-defined constants are available throughout the script and their values ​​remain unchanged during script execution.

The following is a sample code that demonstrates how to define and use user-defined constants:

define("CUSTOM_CONSTANT", "Hello, World!");

echo CUSTOM_CONSTANT;

3. Differences and connections

• Scope: Predefined constants are globally available and can be used anywhere in the program, and users Defined constants are also globally available, but need to be defined anywhere in the script using the define() function.
• Speciality: Predefined constants are some special constants defined internally in the PHP language, such as __FILE__, __LINE__, etc. User-defined constants are constants defined by programmers according to their needs.
• Cannot be modified: The value of a predefined constant cannot be modified, and the value of a user-defined constant cannot be modified after it is defined.
• Contact: Predefined constants and user-defined constants are identifiers that are regarded as constant values ​​and play a fixed value role in programming.

Through the explanations and code examples of this article, we can more clearly understand the differences and connections between predefined constants and user-defined constants in PHP. Predefined constants are special and cannot be modified, while user-defined constants are programmer-defined constant values. In actual programming, choosing to use different types of constants as needed can improve the readability and maintainability of the program.

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