没有名字的变量：$ {''}

So variable variables are existing. Meaning that this is working

$a = 'test';
$$a = 'Hello';
echo ${'test'}; //outputs 'Hello'

But now I've come across some rather strange code using a variable without a name:

function test(&$numRows) {
    $numRows = 5;
    echo ' -- done test';
}

$value = 0;
test($value);
echo ' -- result is '.$value;

test(${''}); //variable without name

http://ideone.com/gTvayV Code fiddle

Output of this is:

-- done test -- result is 5 -- done test

That means, the code is not crashing.

Now my question is: what exactly happens if $numRows value is changed when the parameter is a variable without name? Will the value be written into nirvana? Is that the PHP variable equivalent to /dev/null? I wasn't able to find anything specific about this.

Thanks in advance

Now my question is: what exactly happens if $numRows value is changed when the parameter is a variable without name?

There's no such thing as a variable without name, an empty string in PHP is a totally valid name.

Maybe I'm wrong, but in PHP, all varibles can be accessed by their names (or more precisely, the string representation of their name), and since an empty string is still a string, it counts as a valid name.

Think about variables like an array key-value pair. You can create an array key with an empty string:

$arr = [];
$arr[''] = 'appul';
var_dump($arr['']); // prints: string(5) "appul"
$arr[''] = 'ponka';
var_dump($arr['']); // prints: string(5) "ponka"

Whenever you access $arr[''], you address the same value.

You can access all variables as a string using the $GLOBAL variable too, so you can examine what happens to your "nameless" variable:

${''} = 'ponka';
var_dump($GLOBALS['']); // prints: string(5) "ponka"
${''} = 'appul';
var_dump($GLOBALS['']); // prints: string(5) "appul"

Will the value be written into nirvana? Is that the PHP variable equivalent to /dev/null? I wasn't able to find anything specific about this.

No, it doesn't go to nirvana, it sits quietly in the global space, and it's a little bit trickier to access it, but otherways, it's a normal variable like any others.

${''} is a valid variable which name happens to be an empty string. If you have never set it before, it is undefined.

var_dump(isset(${''}));   // if you have never set it before, it is undefined.

You don't see any error because you disabled the NOTICE error message.

error_reporting(E_ALL);
ini_set('display_errors', 1);

echo ${''}; // Notice: Undefined variable:

You can set it like this:

${''} = 10;
echo ${''};  // shows 10