在关联数组内搜索和替换

我需要在关联数组内搜索和替换。

ex:

$user = "user1"; // I've updated this

$myarray = array("user1" => "search1", "user2" => "search2", "user3" => "search1" ) ;

我想更换search1 for search4。我怎样才能实现这个目标？

更新：我忘了提到该数组有几个search1值，我只想更改其中的值key is == $user。抱歉没有早点提到这一点。

$myarray = array("user1" => "search1", "user2" => "search2" );

foreach($myarray as $key => $val)
{
    if ($val == 'search1') $myarray[$key] = 'search4';
}