Description:
------------
In the past, to call a variable as a function, the variable had to be a string: $var = 'strlen'; $var(/*args*/); calls strlen.

But in the present, the message is a bit misleading, since it must not necessarily be a string: $foo = function() {echo "bar";};

I consider this to be a very minor thing, but consider the message wrong, anyway.

Reproduce code:
---------------
function badLambdaCreator($argument) {
    function($test) use ($argument) { 
        /*Do something*/
    };
    /*No return given :O*/
}
$foo = badLambdaCreator(10);
$foo(50); 
//Fatal error telling you to use strings, 
//while a lambda is fine too.

Expected result:
----------------
A fatal error that also includes lambdas (I guess callbacks work? I'm not sure what name is the most appropiate).

Actual result:
--------------
An error that tells you that **ONLY** strings can be used.