Checking if a PHP Constant is Defined: The Ultimate Showdown Between `defined()` and `isset()`

## The Scenario When developing PHP applications, especially larger projects like `wiki.lib00.com` that require managing configurations and states, we frequently use the `define()` function to create global constants. For instance, defining a constant to represent the current language: ```php // Define a global constant to store the current language define('CURRENT_LANG', 'en-us'); ``` A crucial step before using this constant elsewhere in the application is to check if it has already been defined. This prevents errors from re-declarations or using an undefined constant. So, what is the best way to perform this check? --- ## The Correct Method: Use the `defined()` Function PHP provides a built-in function specifically for checking if a constant exists: `defined()`. The `defined()` function takes a single string argument, which is the name of the constant you want to check. It returns `true` if the constant is defined, and `false` otherwise. This is the recommended and safest approach. **Code Example:** ```php // Let's assume CURRENT_LANG might have been defined in a config file like lib00_config.php // define('CURRENT_LANG', 'zh-cn'); if (defined('CURRENT_LANG')) { // If the constant exists echo "Constant 'CURRENT_LANG' is defined with value: " . CURRENT_LANG; } else { // If the constant does not exist, we can set a default value define('CURRENT_LANG', 'en-us'); echo "Constant 'CURRENT_LANG' was not defined, set to default: " . CURRENT_LANG; } ``` This pattern is excellent for setting up default configurations and ensuring code robustness. --- ## The Common Pitfall: Why Not Use `isset()`? Beginners might instinctively reach for `isset()` to check for a constant's existence because it's commonly used for variables. However, this is the **incorrect** approach for constants. `isset()` is designed to determine if a **variable** is declared and is not `null`. When you pass an undefined constant name to `isset()`, here's what happens: 1. PHP tries to resolve `CONSTANT_NAME` as a constant. 2. When it fails, PHP assumes it's a string literal `'CONSTANT_NAME'`. 3. This process triggers an `E_NOTICE` level error: `Notice: Use of undefined constant CONSTANT_NAME - assumed 'CONSTANT_NAME'`. 4. `isset()` then checks this string of the same name, which always evaluates to `true`, completely defeating the purpose of the check. **Incorrect Example:** ```php // Let's ensure the constant is not defined for this test if (isset(UNDEFINED_CONSTANT)) { // This code block will execute, but with a Notice echo 'isset() returned true, but this is a misleading result.'; } // In PHP > 7.2, this Notice is elevated to a Warning // Warning: Use of undefined constant UNDEFINED_CONSTANT - assumed 'UNDEFINED_CONSTANT' ``` --- ## Core Differences: `defined()` vs. `isset()` | Feature | `defined('CONSTANT_NAME')` | `isset($variable_name)` | `isset(CONSTANT_NAME)` | | :--- | :--- | :--- | :--- | | **Purpose** | **Checks if a constant exists** | Checks if a variable is set and not `null` | **Incorrect usage** | | **Argument** | The **string name** of the constant | A variable | An undefined constant name | | **Behavior** | Safely returns `true` or `false` | Safely returns `true` or `false` | Triggers a `Notice`/`Warning` and gives a faulty result | --- ## Conclusion To write clean, error-free, and professional PHP code, always remember this golden rule: > **To check for constants, always use `defined()`.** This simple practice will help you avoid debugging headaches and potential logic flaws. Make sure to follow this best practice in your next project, for instance, when contributing to the `wiki.lib00` codebase. -- DP@lib00