$object->{$arryName}[] = "test"
The curly braces change the order of operations and will make PHP evaluate the variable name before the hard braces.
If you want to do a associative array, this gets a little more complicated:
$object->{$arryName}[$keyname] = "test"
In this case, you can put curly braces around $keyname but it's entirely optional.
On a related note.. variable variables are usually - but not always - a sign of something screwy. They're also a pain to whoever is coming after you who has to debug, refactor, do any grepping, etc. If you must use them, fine but make sure you've considered the ramifications.