php Anonymous function

Question

When I was reading questions for Zend Certified PHP Engineer 5.5 I saw question about anonymous function but I need to explan how it work.

function z($x)
{
    return function($y) use ($x)
    {
        return str_repeat( $y , $x );
    };
}

$a = z(2);
$b = z(3);

echo $a(3).$b(2);

The output for this code is:

33222

But in function header there is only $x parameter from where $y got there value!

Answer 1

Function z creates and returns a new function, but an anonymous one. That new function is defined so that it has one argument - $y. However, this anonymous function also uses argument $x from a function z.

To make it simple, function z basically creates a function which can repeat any string, but a fixed number of times. The number of times a string is repeated is determined by the value of argument $x in z.

So, calling z(2) creates a new function which is functionally equivalent to writing

function repeat_two_times($y) {
    return str_repeat($y, 2);
}

In you example, hard coded value 2 is determined by the value of $x.

You can read more about this in the documentation. The principle displayed by the example can be quite useful for creating partial functions like add5, inc10, ...

Answer 2

Firstly, you initialize function z:

$a = z(2);

$x in the example is set to 2, so the returned function (anonymous function, also called closure) can now be read as (because $x is used):

$a = function($y) {
    return str_repeat($y, 2);
}

When invoking this function:

echo $a(3);

You are supplying this return function with the parameter 3 ($y).

The output is: 33

Answer 3

Anonymous functions are also known as Closures.

You ask where $y gets its value. The code example is difficult to decipher because you use 2s and 3s everywhere. Things would be clearer if your last lines were

$a = z(2);
$b = z(3);
echo $a('A').$b('B');

That would result in:

AABBB

But let's follow your code. Notice that there are two related function calls

 $a = z(2);

and

 echo $a(3);

calling function z() with argument 2 returns a function (that is assigned name $a) where line

 return str_repeat($y, $x);

is in reality :

 return str_repeat($y, 2);

now, you call that function $a() with argument 3. That 3 (value of $y) is repeated two times

The same analysis applies to the other related function calls:

 $b = z(3);
 ...
 echo ... $b(2); 

But in this case 2 is repeated 3 times

Answer 4

 function z($x)
  {
    return function($y) use ($x)
    {
        return str_repeat( $y , $x );
    };
  }

  $a = z(2);// here you are setting value of x by 2
  $b = z(3);// here you are setting value of x by 3
echo $a(3).$b(2);// here $a(3) 3 is value of y so it becomes str_repeat( 3 , 2 ); which is 33