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What are the best practices for checking the existence and value of constants in PHP to avoid errors and ensure proper functionality?

When working with constants in PHP, it is important to check their existence and value before using them in order to avoid errors and ensure proper functionality. One way to do this is by using the defined() function to check if a constant is defined, and then using the constant() function to get its value if it exists.

php 
// Check if a constant is defined and get its value
if (defined('MY_CONSTANT')) {
    $value = constant('MY_CONSTANT');
    // Use the constant value here
    echo $value;
} else {
    // Handle the case when the constant is not defined
    echo 'Constant MY_CONSTANT is not defined';
}

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