How does the "&" operator work in a PHP function?

Please see this code:

function addCounter(&$userInfoArray) {
    $userInfoArray['counter']++;
    return $userInfoArray['counter'];
}

$userInfoArray = array('id' => 'foo', 'name' => 'fooName', 'counter' => 10);
$nowCounter = addCounter($userInfoArray);

echo($userInfoArray['counter']);

This will show 11.

But! If you remove "&"operator in the function parameter, the result will be 10.

What's going on?

Answer

The & operator tells PHP not to copy the array when passing it to the function. Instead, a reference to the array is passed into the function, thus the function modifies the original array instead of a copy.

Just look at this minimal example:

function foo($a) { $a++; }
function bar(&$a) { $a++; }

$x = 1;
foo($x);
echo "$x\n";    
bar($x);
echo "$x\n";

?>

Here, the output is:

1
2

– the call to foo didn’t modify $x. The call to bar, on the other hand, did.