通过引用返回PHP

In documentation i see how we set 2 &, why?

And can say me please what difference beetween

this 1:

function &func()
{
     static $static = 0;
     $static++;
     return $static;
}
$var1 = func();
echo "var1:", $var1; // 1

and this 2:

function func()
{
     static $static = 0;
     $static++;
     return $static;
}
$var1 = func();
echo "var1:", $var1; // 1

Or alternative variant

this 1:

function &func()
{
     static $static = 0;
     $static++;
     return $static;
}
$var1 = &func();
echo "var1:", $var1; // 1

and this 2:

function func()
{
     static $static = 0;
     $static++;
     return $static;
}
$var1 = &func();
echo "var1:", $var1; // 1

There is only a difference between the first this 2 and the second this 1. All others are wrongly tried return by reference.

The second this 2 even throws a PHP notice (Notice: Only variables should be assigned by reference in ... on line ...).

The difference is that the first this 2 returns by value while the second this 1 returns by reference. Since it is a reference, the variable can be changed outside the function.

function &func()
{
     static $static = 0;
     $static++;
     return $static;
}
$var1 = &func();
echo "var1:", $var1; // 1
$var1 = 10;
$var1 = &func();
echo "
var1:", $var1; // 11

https://3v4l.org/uJHSF

evals for all snippets (with second function call):
1st this 1: https://3v4l.org/p6erT
1st this 2: https://3v4l.org/9OcaC
2nd this 1: https://3v4l.org/uJHSF
2nd this 2: https://3v4l.org/qJ3qO