Are there any potential pitfalls to be aware of when manipulating webpage source code with PHP?

One potential pitfall when manipulating webpage source code with PHP is introducing security vulnerabilities, such as cross-site scripting (XSS) attacks. To mitigate this risk, it is important to properly sanitize user input and validate any data before outputting it to the webpage.

// Example of sanitizing user input before outputting to webpage
$userInput = $_POST['input'];
$sanitizedInput = htmlspecialchars($userInput, ENT_QUOTES, 'UTF-8');
echo $sanitizedInput;