In PHP, constants are identifiers (names) that represent fixed values that cannot be changed during the execution of a script. They provide a way to define values that remain consistent throughout your code. Constants are often used for configuration settings, predefined values, and other data that should not be modified.

Here's how you can define and use constants in PHP:

Defining Constants:

Constants are defined using the `define()` function or the `const` keyword.

Using the `define()` function:
php
define("PI", 3.14159);
define("APP_NAME", "My Application");

Using the `const` keyword (only available in class definitions):
php
class ConstantsClass {
    const PI = 3.14159;
    const APP_NAME = "My Application";
}

Using Constants:

You can use constants just like variables, but you don't need to use the dollar sign (`$`) before the constant name.

php
echo PI; // Output: 3.14159
echo ConstantsClass::PI; // Output: 3.14159 (if using the class-based constant)
echo APP_NAME; // Output: "My Application"
echo ConstantsClass::APP_NAME; // Output: "My Application" (if using the class-based
constant)

Benefits of Constants:

1. Readability: Constants make your code more readable by giving meaningful names to fixed values, improving code understanding.
2. Code Maintenance: Constants provide a central place to modify values that might change, making it easier to update your code.
3. Prevent Mistakes: Using constants helps prevent accidental modification of important values.
4. Global Availability: Constants are globally available in the script, so you don't need to pass them around like variables.
5. Performance: Constants are evaluated at compile time, which can result in better performance compared to variables.

Conventions:

- Constants are typically defined in uppercase letters.
- It's common to use underscores (`_`) to separate words in constant names (e.g., `APP_NAME`).
- Constants can be used in functions, classes, and throughout your code.

Keep in mind that constants cannot be changed or reassigned once defined, so they are suitable for values that should remain constant throughout the execution of your script.

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