Rotate an associative array so that it begins with a nominated key

Question

I was working with an array that I need to manipulate maintaining the keys. The array_shift() function, which I need to implement, unfortunately does not maintains the key of the processed element of the array. Please see the example code below:

$rotor = array ("A" => "R", "B" => "D", "C" => "O", "D" => "B");

$position = "C";

foreach ($rotor as $key => $value) {
    if ($key != $position) {
        $temp = array_shift($rotor);
        array_push($rotor, $temp);
    } else {
        break;
    }
}
var_dump($rotor);

Result

array(4) {
 ["C"]=>
 string(1) "O"
 ["D"]=>
 string(1) "B"
 [0]=>
 string(1) "R"
 [1]=>
 string(1) "D"
}

As you can see, the keys of the R and D elements aren't the original ones. What alternative solution do you recommend to extract a first element of an array keeping unchanged the key?

P.S: My goal is to extract the first element of the array (keeping the key), and if it is not equivalent to the variable $position, insert the element itself in the last position of the array always maintaining its key.

Answer 1

$rotor = array ("A" => "R", "B" => "D", "C" => "O", "D" => "B");
$position = "C";

foreach ($rotor as $key => $value) {
if($key != $position) {
    array_shift($rotor);
    $rotor += array($key=>$value);
} else {
    break;
}
}

Try this - works for me.

Answer 2

An alternative technique which may perform better or worse depending on the volume of data and the location of the new starting element involves slicing the second half from the array and replacing the original array onto the end. This effectively places the earlier occurring keys to the back of the array.

Code: (Demo)

$rotor = ["A" => "R", "B" => "D", "C" => "O", "D" => "B"];
$from = "C";

foreach (array_keys($rotor) as $index => $key) {
    if ($key === $from) {
        $rotor = array_slice($rotor, $index, null, true) + $rotor;
        break;
    }
}
var_export($rotor);

Output:

array (
  'C' => 'O',
  'D' => 'B',
  'A' => 'R',
  'B' => 'D',
)

This only ever make 1 function call within the loop instead of calling array_shift() over and over until the sought key is encountered.

This technique is safe/reliable even if the sought key is in the first element or doesn't occur in the array at all.