php call standard function by string

I need to call a PHP standard function from a string

开发者_开发知识库

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.

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$functionname($param);

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Correction, ok I thought this would work:

$$functionname($param);

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