Pure functions in php

As per wiki, pure function is the one with following characteristics:

• Its return value is the same for the same arguments (no variation with local static variables, non-local variables, mutable reference arguments or input streams from I/O devices).
• Its evaluation has no side effects (no mutation of local static variables, non-local variables, mutable reference arguments or I/O streams).
  Thus a pure function is a computational analogue of a mathematical function. Some authors, particularly from the imperative language community, use the term “pure” for all functions that just have the above property.

I am trying to translate that to PHP. Here are the thoughts I have on pure functions:

• All arguments are passed by value and not by reference.
  • No arguments with &
  • No objects passed as arguments as objects are passed by reference
• They don’t access state of the class
  • They don’t access anything with $this
• The don’t access global or super globals
  • They don’t use $_ variables
  • They don’t have global keyword
• They don’t access I/O, network calls, etc.
  • No file operations
  • No network calls, database calls, etc.
• They don’t have static calls
  • When there are static calls, their results may change
• There are no side effects
  • State of the class shouldn’t change, i.e., it should’t update members of the class
• Output is solely dependent on input

Let me know your thoughts and correct me if I am missing something.