How can user input manipulation in PHP scripts lead to external control of file names or paths?

User input manipulation in PHP scripts can lead to external control of file names or paths if the input is not properly sanitized or validated. Attackers can inject malicious file names or paths into the script, potentially allowing them to access sensitive files on the server or execute arbitrary code. To prevent this, always validate and sanitize user input before using it to construct file names or paths.

// Sanitize user input before using it in file operations
$userInput = $_POST['filename'];
$cleanedInput = filter_var($userInput, FILTER_SANITIZE_STRING);

// Use the sanitized input to construct file paths
$filePath = '/path/to/files/' . $cleanedInput;