Why is it important to use the defined() function instead of isset() when checking if constants are set in PHP?

When checking if constants are set in PHP, it is important to use the `defined()` function instead of `isset()` because `isset()` will return false for constants that are defined but have a value of `null`. The `defined()` function specifically checks if a constant has been defined, regardless of its value. 

// Incorrect way using isset()
if (isset(CONSTANT_NAME)) {
    echo 'Constant is set';
} else {
    echo 'Constant is not set';
}

// Correct way using defined()
if (defined('CONSTANT_NAME')) {
    echo 'Constant is set';
} else {
    echo 'Constant is not set';
}

Keywords

defined() isset() constants PHP error types

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