php Anonymous function - php

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