How can constant values be efficiently used in PHP to improve code readability and maintainability?

Constant values can be efficiently used in PHP by defining them at the beginning of the code or in a separate file. By using constants, you can assign meaningful names to values that are used repeatedly throughout the code, improving readability and making it easier to update these values in one central location. This approach also helps in maintaining the code as it reduces the chances of errors due to mistyped values. 

// Define constants for commonly used values
define('TAX_RATE', 0.10);
define('MAX_ATTEMPTS', 3);

// Usage of constants in code
$taxAmount = $price * TAX_RATE;
if ($attempts >= MAX_ATTEMPTS) {
    echo "Maximum attempts reached.";
}

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