Execute from var?

I am experimenting with Execute function.

This way- works fine for me

$a = "fileread"
$f = Execute ('fileread ("test.txt")')

ConsoleWrite ($f)

But  if I replace fileread with variable $a then it wont work

$a = "fileread"
$f = Execute ('$a ("test.txt")')

ConsoleWrite ($f)

Is there a way to make it work from var?

Great thanx in advance!