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.)