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English
In PHP, the define function is used to declare constants. Once defined, the value of a constant cannot be changed. Constants are usually named in uppercase letters to distinguish them from variables. Using constants improves code maintainability and readability, while preventing accidental modification of values, ensuring program stability. Next, we will explore the functionality and common uses of the define function with code examples.
You can define constants using the define function. The basic syntax is:
define('CONSTANT_NAME', 'value');
Constant names are typically uppercase, and the value can be of any data type. For example, defining a constant named PI with a value of 3.14:
define('PI', 3.14);
After defining a constant, you can use it anywhere in your PHP script. Accessing a constant is straightforward: just use its name directly. For example, using the previously defined PI constant:
echo PI; // outputs 3.14
Before using a constant, you can check if it has already been defined using the defined function. The syntax is as follows:
defined('CONSTANT_NAME');
This function returns a boolean value: true if the constant is defined, false otherwise. Here's an example checking if the PI constant is defined:
if (defined('PI')) {
echo 'PI is defined';
} else {
echo 'PI is not defined';
}
In PHP, constants have a global scope, meaning they can be accessed anywhere in the script. Constants are not limited by functions or classes, so they can be used inside or outside functions. For example, accessing the PI constant inside a function:
function displayPi() {
echo PI;
}
displayPi(); // outputs 3.14
PHP provides many predefined constants that can be used directly in scripts without needing to be defined via define. For example, __FILE__ represents the current file path, and __LINE__ represents the current line number. Example usage of predefined constants:
echo __FILE__; // outputs current file path
echo __LINE__; // outputs current line number
The define function allows PHP developers to create constants whose values cannot be changed. Constants have a global scope, accessible anywhere in the script without restrictions from functions or classes. The defined function helps check if a constant exists before usage. PHP also offers numerous predefined constants that can be used without explicit definition.
Mastering the use of the define function helps write more stable and efficient PHP code.
