如何交换数组中的两个元素？

您如何在代码中交换“firstname”（“Martin”）和数组名称（“Krönström”），或者更确切地说，如何在第一个 foreach 循环中为 赋值$firstname = $lastnames[$key]['firstname'];，然后在 中使用它$newlastnames[$firstname]["firstname"] = $key;？

请解释什么，分配在funcion a哪里？

function a ($lastnames){
    $newlastnames;
    foreach($lastnames as $key => $value){
        $firstname = $lastnames[$key]['firstname'];
        $newlastnames[$firstname]["firstname"] = $key;
        $newlastnames[$firstname]["age"] = $lastnames[$key]['age'];
        $newlastnames[$firstname]["sex"] = $lastnames[$key]['sex'];
        echo $lastnames[$key]['firstname'].'<br>';
   }
    return $newlastnames;
}

整个代码在这里

$lastnames = array(

"Krönström" => array("firstname" => "Martin",     "age" => 20, "sex" => "male"),
"Kulper"    => array("firstname" => "Laura-Liis", "age" => 17, "sex" => "female"),
"Ööbik"     => array("firstname" => "Tõnis",      "age" => 21, "sex" => "male"),

"Mitri"   => array("firstname" => "Martten", "age" => 35, "sex" => "male"),
"Lõsenko" => array("firstname" => "Evelina", "age" => 16, "sex" => "female"),
"Reinaus" => array("firstname" => "Richard", "age" => 21, "sex" => "male"));

$newlastnames =  a($lastnames);
saveFile($newlastnames);

function saveFile($lastnames){
    file_put_contents("q.json",json_encode($lastnames));
}


function a ($lastnames){
    $newlastnames;
    foreach($lastnames as $key => $value){
        $firstname = $lastnames[$key]['firstname'];
        $newlastnames[$firstname]["firstname"] = $key;
        $newlastnames[$firstname]["age"] = $lastnames[$key]['age'];
        $newlastnames[$firstname]["sex"] = $lastnames[$key]['sex'];
        echo $lastnames[$key]['firstname'].'<br>';
   }
    return $newlastnames;
}
function sortA($lastnames,$sortValue){
    foreach($lastnames as $key => $value){
        if($sortValue == "firstname"){
            $throwArray[$key] = $key;
        }else{
            $throwArray[$key] = $value[$sortValue];
        }
    }
    array_multisort($throwArray, SORT_DEC, $lastnames);
    return $lastnames;
}