php call standard function by string

I need to call a PHP standard function from a string

Example:

$param = "parameter";
$functionname = "createimagefromjpg";
//call the function (with a parameter)

Anyone has an idea how I can do this? It can't be done with call_user_func() because it is a standard function.

$functionname($param);

Correction, ok I thought this would work:

$$functionname($param);

but it does not.... the example with one dollar sign works for functions. Nice.