result of prepared select statement as array

I would like to get the complete result of a prepared statement as an array (key/value pairs) in order to later use it in a str_replace() function.

My table has three columns, an index and the fields "x1" and "x2". I used the following successfully:

$db = new mysqli("servername", "username", "pw", "dbname");

if($ps1 = $db->prepare("SELECT x1, x2 FROM my_table")) {
  $ps1->execute();
  $ps1->bind_result($search, $replace);
    $result = array();
    while ($ps1->fetch()) {
      $result[$search] = $replace;
    }
    $ps1->close();
}

However, I am thinking that there must be a simpler way, without a while loop, getting the complete result, not added up from single rows one by one.

I looked at other questions, and I came up with the following, but it doesn't work ("Warning: mysqli_fetch_assoc() expects parameter 1 to be mysqli_result"):

if($ps1 = $db->prepare("SELECT x1, x2 FROM my_table")) {
  $ps1->execute();
  $result = mysqli_fetch_assoc($ps1);
  return $result;
  $ps1->close();
}

I also tried $result = mysqli_fetch_all($ps1); with no success ( getting "Call to undefined function mysqli_fetch_all()").

BTW, I am using PHP 5.6.

from result of prepared select statement as array