In PHP, what are some common methods for converting user input into safe and usable HTML content, especially when dealing with links and special characters?

When dealing with user input that will be displayed as HTML content, it is important to sanitize the input to prevent potential security vulnerabilities such as cross-site scripting (XSS) attacks. One common method to achieve this is by using the `htmlspecialchars()` function in PHP, which converts special characters like `<`, `>`, `&`, and `"` into their HTML entity equivalents. Additionally, when dealing with user-provided links, it is recommended to use the `filter_var()` function with the `FILTER_VALIDATE_URL` filter to validate and sanitize the URL input.

// Sanitize user input for HTML content
$userInput = "<script>alert('XSS attack');</script>";
$safeHtmlContent = htmlspecialchars($userInput, ENT_QUOTES);

echo $safeHtmlContent;

// Validate and sanitize user-provided link
$userLink = "https://example.com";
if (filter_var($userLink, FILTER_VALIDATE_URL)) {
    $safeLink = filter_var($userLink, FILTER_SANITIZE_URL);
    echo $safeLink;
} else {
    echo "Invalid URL";
}