Declarations

Variable names and other user-defined symbols are known as identifiers, which may be of any length, with upper and lower case being distinct. Identifiers must start with a letter and then be followed by letters, digits or underscores. All unicode characters may be used, as long as your editor saves the source files as utf-8 - note however that all non-ascii characters are treated as letters, including a few that might look rather similar to operators, and even some numbers.
The following reserved words have special meaning in Phix and may not be used as identifiers:
and or not xor break continue exit fallthru case default format fallthrough constant enum abstract iff try catch dynamic iif class struct extends do while until switch for by to end if then elsif else true false null goto function procedure type return ifdef elsifdef elsedef private forward global export public include with without
Edita displays these words in teal (easily changed in Options/Colours).
Since version 0.8.0 the following are also treated as reserved words:
abort atom number bool close compare int integer seq sequence string object delete equal floor flush log peek poke power prepend profile trace seek sin sleep version where
Identifiers can be used in naming the following:

Note however that none of the following (and then some) are technically reserved words, although the editor colours them differently, to give you a hint that it could be rather foolish to use them as identifiers:
apply assert boolean call choose crash date elapsed factors filter find free head join largest listen lower map match max min mod pad print product proper progress repeat reverse shorten shuffle sign smallest sort split substitute sum system tail trim union unique upper value
Also, do yourself a favour and avoid these JavaScript reserved words, should you plan on ever using pwa/p2js to run stuff in a browser:

arguments await const debugger delete eval final finally implements import in instanceof interface let native new package protected super synchronized this throws transient typeof var void volatile yield
The transpiler issues a fatal error should you try to use any of those. A few more have also slipped in that trigger strictly unnecessary errors, some of which I might be able to stop the transpiler from issuing, and some of which will have to be added to the above table. (I basically used the same mechanism for syntax colouring and reserved words, oops.)