PHP: How to solve ob_start() in combination imagepng() issue?

php output-buffering

PHP Snippet 1:

imagepng($imgSignature, 'php://memory/file.png');
$base64Signature = base64_encode(file_get_contents('php://memory/file.png'));

PHP Snippet 2:

ob_start();
imagepng($imgSignature);
imagedestroy($imgSignature);
$data = ob_get_contents();
ob_end_clean();
$data = base64_encode($data);