This function can be used before saving a file to make sure it has a filename that is sanitized and unique for the given directory.
If the filename is not unique, then a number will be added to the filename before the extension, and will continue adding numbers until the filename is unique.
The function wp_unique_filename is passed three parameters:
The callback function allows the caller to use their own method to create unique file names.
If defined, the callback should take three arguments - directory, base filename, and extension - and return a unique filename.
$newfilename = wp_unique_filename( $path_being_saved_to, $filename_to_check );
wp_unique_filename() is located in