Description:
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Assuming an object scope, where $info['callback'] = 'callbackMethod' and 'callbackMethod' is a method on $this object.

The following code works under 5.6.0:
####
$out = $this->$info['callback']($data);
####

But in Master it throws a fatal error (Function name must be a string).
Changing it to the following and it again works as expected:
####
$out = $this->{$info['callback']}($data);
####
Unfortunately this code is from another vendor, so I cannot expect them to change this although using curly braces may be better practice in general to specify the proper evaluation order.

Expected result:
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No fatal error.