What is closure in KCacheGrind?

what is the meaning of closure in KCacheGrind? I have it with one of my functions and it is pointing out the spl_autoload_register() function, spl_autoload_call in KCacheGrind. And the self time of the function is 60+ so, of course, I want to optimize it, but I do not know where to start.

What is the closure in KCacheGrind?

What do I need to optimize the said function to lessen the self time?

A closure is a function that uses variables that are outside its local scope, but not global.

I'll use a language agnostic example since it's been forever since I've written PHP:

function someFunc() {
    var a = 0;

    return function() { // This is the closure
        a++;
        return a;
    } 
} 

var f = someFunc();

print(f()); // Prints 1
print(f()); // Prints 2
print(f()); // Prints 3

Note the first comment. That function being returned is a closure over the a variable.