What are common pitfalls when adapting existing PHP code for customization?

One common pitfall when adapting existing PHP code for customization is not properly understanding the original code structure, leading to unintended consequences when making changes. To avoid this, it's important to thoroughly analyze the existing code and document its functionality before making any modifications. Additionally, not testing the customized code thoroughly can result in unexpected errors or bugs.

// Example of adapting existing PHP code for customization

// Original code snippet
function calculateTotal($price, $quantity) {
    return $price * $quantity;
}

// Customized code snippet
function calculateTotalWithTax($price, $quantity, $taxRate) {
    $total = $price * $quantity;
    $taxAmount = $total * $taxRate;
    return $total + $taxAmount;
}