sqlite "database is locked error" on insert data with ajax call

i try to do 2 ajax call with php and jquery that insert data into my sqlite database, the first insert call works fine,but when the second call start,the server respond: database is locked

the two ajax requests start at the same time,They are simultaneous.

how i can solve it?

here's my php code:

<?php
class MyDB extends SQLite3
{
  function __construct()
  {
     $this->open('../copiaDatabase.sqlite');
  }
}
$db = new MyDB();
if(!$db){
  echo $db->lastErrorMsg();
} else {
    echo "Opened database successfully\n";
}


$opera=$_POST["opera"];
$codiceBeacon=$_POST["codice"];
$museo= (string)$_POST["museo"];

$codice_qr_random=$_POST["codice_qr_random"];




$sql = <<<EOF
  INSERT INTO 'selectedObject' (museum , atwork , beaconCode , qrCode)
  VALUES ('$museo' , '$opera', '$codiceBeacon' , '$codice_qr_random');
 EOF;



$ret = $db->exec($sql);
 if(!$ret){
    echo $db->lastErrorMsg();
  } else {
  echo "Records created successfully\n";
  }

  $db->close();

  ?>

and this is my ajax call using jquery:

 $.ajax({

           type: "POST",
           url: "db/liteVersionPhpApplicazione/inserimentoCodiceLite.php",
           // data:{opera: opera, codice: codice},
           data: {
              opera: opera,
              codice: codice,
              museo: museo,
              codice_qr_random: codice_qr_random
           },
           success: function (data) {
                 alert(data);
           }
        })

     });