How important is it to implement input validation and sanitization in PHP scripts to prevent unexpected behavior like writing unwanted characters to a file?

It is crucial to implement input validation and sanitization in PHP scripts to prevent unexpected behavior like writing unwanted characters to a file. This helps to ensure that only valid and safe data is processed, reducing the risk of security vulnerabilities such as injection attacks.

// Example of input validation and sanitization in PHP
$input = $_POST['user_input']; // Assuming user input is coming from a form

// Validate and sanitize the input
$clean_input = filter_var($input, FILTER_SANITIZE_STRING);

// Write the sanitized input to a file
$file = fopen("output.txt", "w");
fwrite($file, $clean_input);
fclose($file);