What potential pitfalls should be considered when manipulating strings in PHP for file naming conventions?

When manipulating strings in PHP for file naming conventions, potential pitfalls to consider include ensuring the file name is unique, handling special characters that may not be allowed in file names, and properly sanitizing user input to prevent security vulnerabilities.

// Example of sanitizing a string for use as a file name
$filename = "my file name";
$filename = preg_replace("/[^a-zA-Z0-9\_\-\.]/", "", $filename); // Remove any characters that are not letters, numbers, underscores, hyphens, or periods
$filename = strtolower($filename); // Convert the file name to lowercase