What can be used in Computed Code?

It is my understanding that php code in the Computed Code text area is to be executed by the module.

My example is simplest of php yet I do not understand while it will not work.

If i place an integer in $node_field[0]['value'] = ""; such as

$node_field[0]['value'] = "4";

It will not display 4 in my node

However if I alternatively do this in the Display Field

$display =4;

then it will display

what is confusing me is that

$display = $node_field_item['value']; should produce the value in the Computed Field $node_field[0]['value'] = "";

why does the example above not work?

$node_field[0]['value'] = "4"; should cause $display = $node_field_item['value']; to be equal to 4