It's less fast than direct lookups, but if the compiler supports it properly, it shouldn't be any slower than a associative array lookup.

  echo $FOO[$BAR];
  echo $$BAZ;
  >>>
  
  push FOO.ptr
  push BAR.ptr
  call hash_get
  push ax
  call print
  
  push local_table // static per compiled scope
  push BAZ.ptr
  call hash_get
  push [ax+24] // entry->value
  call print

You'd probably want to add error handling (to both cases) and accessing a global variable that didn't exist when the code was compiled will likely be slow, but if becomes a preformance bottleneck you can always rewrite it to something easier to compile.