How do PHP static keyword work?

Question

I'm not sure when should I use the static keyword and I don't know where can I use that.
I found the below code example in a PHP challenge -
function myFunction($i=1) {
static $id = 101;
$id -= $i;
echo $id;
}
myFunction();
echo"\n";
myFunction(1);
echo"\n";
myFunction(-1);
// output = 100 99 100

I didn't Google for it. I want to learn this from SoloLearn family 💜

Answer 1

The PHP keyword static in functions allows you to define a variable which will maintain its value every time you call that function.

The first time $id is assigned to 101 and then the function subtracts 1 ($id is now 100).
The second time $id is already assigned (100), so the static assignment is ignored and the function subtracts again 1 ($id is now 99).
The third time $id is 99 and the function subtracts -1, thus adding 1 ($id is now 100).