What are the best practices for validating and sanitizing user input when dealing with SimpleXML in PHP?

When dealing with user input in SimpleXML in PHP, it is important to validate and sanitize the input to prevent potential security vulnerabilities such as XML injection attacks. One way to do this is by using PHP's filter_input() function to validate the input and htmlentities() function to sanitize it before using it in SimpleXML. 

// Validate and sanitize user input for SimpleXML
$input = filter_input(INPUT_POST, 'user_input', FILTER_SANITIZE_STRING);
$sanitized_input = htmlentities($input);

// Load the sanitized input into SimpleXML
$xml = simplexml_load_string($sanitized_input);

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