Use strict checks: is_file() and/or is_link() instead of file_exists() before attempting unlink()
Remove redundant file_exists() if is_readable() is also used.
Use is_readable() check instead of insufficient file_exists() for config includes.
Use is_readable() and file_exists() checks instead of file_exists() for backup file.
More strict check.
Use is_dir() instead of file_exists() when checking directory existence.
Use is_readable() instead of file_exists() when checking alias existence.