What are the potential security risks of manipulating HTML elements using PHP in a web application?

One potential security risk of manipulating HTML elements using PHP in a web application is the possibility of introducing cross-site scripting (XSS) vulnerabilities. To mitigate this risk, it is essential to properly sanitize user input before outputting it to the HTML document. This can be achieved by using functions like htmlspecialchars() to encode special characters and prevent them from being interpreted as HTML or JavaScript code by the browser.

// Sanitize user input before outputting to HTML document
$userInput = "<script>alert('XSS attack!')</script>";
$sanitizedInput = htmlspecialchars($userInput, ENT_QUOTES, 'UTF-8');
echo "<p>" . $sanitizedInput . "</p>";