I understand little & big endian, but What's "machine byte order" mean?
In pack the phrase "machine byte order" means that the endianess is determined by the current machine1
PHP itself makes no guarantees as to which endianness such characters (e.g. S, L) encode data, except as the ordering relates to the current machine.
Therefor, be cautious with using "machine byte order" pack characters and consider the guaranteed-order counter-parts (e.g. n, v) if there is every any doubt1. However, pay attention to the target data specification as some silly formats like [Microsoft] UUIDs are laid out in terms of "machine byte order" while others are always big-endian or always little-endian.
1 x86/x64 is always little-endian, but PHP could technically run on big-endian machines .. somewhere. It is best to get into the habit of being explicit and precise to avoid having code suddenly and mysteriously "stop working" later on.