How can i store a base64 image to my site's private folder?

I'm trying to store a base64 image from a Signature field plugin (the plugin outputs a base64 text). But before i can store it to the private folder, i have to first convert it as a png image. This is how i normally do it if i just upload an image:

    // Signature Field on FormBuilder
    $form['signature'] = [
        '#type' => 'managed_file',
        '#title' => $this->t('Signature'),
        '#upload_location' => 'private://requestformfiles',
        '#upload_validators' => [
        'file_validate_extensions' => ['png jpg',],
        ],
    ];

    // Pre-saving
    $image = $form_state->getValue('signature');
    $file = File::load( $image[0] );
    $file->setPermanent();
    $file->save(); 

But with a base64 image, i have to do this first:

    $image = $form_state->getValue('signature');
    $data = base64_decode(preg_replace('#^data:image/\w+;base64,#i', '', $image));
    file_put_contents('storage/signature.png', $data);

My problem is with file_put_contents. The save location is usually done in '#upload_location', but i don't know how to do this in file_put_contents.

Any help is appreciated!