PHP Constants

In PHP, constants are identifiers (names) for simple values. Once a constant is defined, it cannot be changed or undefined. Constants are useful for defining values that should remain consistent throughout your script.

Here’s a basic overview of how to work with constants in PHP:

Defining Constants

You use the define() function to create a constant. The define() function takes two parameters: the name of the constant and its value.

define('SITE_NAME', 'MyWebsite');
define('MAX_USERS', 100);

Accessing Constants

Constants are accessed directly by their name without the need for a $ sign.
echo SITE_NAME; // Outputs: MyWebsite
echo MAX_USERS; // Outputs: 100

Case Sensitivity

By default, constant names are case-sensitive. However, you can make them case-insensitive by setting the third parameter of define() to true.

define('DB_HOST', 'localhost', true); // Case-insensitive
echo DB_HOST; // Outputs: localhost
echo db_host; // Also outputs: localhost

Constants in Classes

In PHP 5.3.0 and later, you can also define constants within classes using the const keyword.

class MyClass {
    const MY_CONST = 'SomeValue';
}
echo MyClass::MY_CONST; // Outputs: SomeValue

Magic Constants

PHP also provides a set of predefined "magic constants" that are automatically defined. These constants include:

    __LINE__ - The current line number of the file.
    __FILE__ - The full path and name of the file.
    __DIR__ - The directory of the file.
    __FUNCTION__ - The name of the function.
    __CLASS__ - The name of the class.
    __METHOD__ - The name of the class method.
    __NAMESPACE__ - The name of the current namespace.

Example of Magic Constants

echo __FILE__; // Outputs the full path of the current file
echo __LINE__; // Outputs the current line number

Constants are a great way to make your code more readable and maintainable by providing meaningful names for values that should not change.