PDO fails by adding a bigger blob file

Question

i have a web-application for uploading PDF-files into a php-system. I get the selected file with

$filedata = base64_encode(file_get_contents($_FILES['upl_file']['tmp_name']));

and defines the connection so

$dbcon = new PDO("mysql:host=localhost;dbname=signshare", "root", "");

The connection does not fail. Now the problem:

When the uploaded pdf-file is small (like <1mb) everything works fine but when the uploaded file is a little bit bigger (testet with ~3.5mb) the PDO-function failed with no error message. The uploaded file is no insert into the database.

This is my way to upload the file as blob:

$insert_file = $dbcon->prepare(
    "UPDATE document SET doc_blob = :blob, `status` = :status WHERE did = :did"
);

$insert_file->bindParam(':blob', $filedata, PDO::PARAM_LOB);
$insert_file->bindParam(':status', $status);
$insert_file->bindParam(':did', $did);

try {
    $insert_file->execute();
} catch (Exception $ex) {
    $logf = fopen('../controller/log.txt', 'a');
    fwrite($logf , "\n BLOB INSERT FAILED: " . $ex . " \n");
    fclose($logf);
}

The programm does not throws the exception.

First I had the error-message "Mysql Server has gone away". Then i set max_allowed_packet to 16M in the my.ini file and upload_max_filesize to 20M in php.ini. The error-message gone but the function still not work.

Thank you for help!