Save Additonal CSS to a file [duplicate]

I am trying to save the Additional CSS to a file not into the database and enqueue it. I created a my.css file in upload folder but stuck in the process to save/ edit the file. I am facing errors in the following code. Any suggestions plz?

add_action('customize_register', 'theme_footer_customizer');
function theme_footer_customizer($wp_customize){
//adding section in wordpress customizer   
$wp_customize->add_section('custom_css', array(
  'title'          => 'New Additional CSS',
  'ID'             => 'custom_css',
  'priority'       =>  200,
 ));
//adding setting for footer text area
$wp_customize->add_setting('custom_css', array(
 'default'        => 'Default Text For Footer Section',
 ));

// Get upload directory
    $upload_dir = wp_get_upload_dir();

    // Create style file path
    $style_file_path = sprintf( '%1$s/css', untrailingslashit( $upload_dir['basedir'] ) );

    // Create directories if they do not exist
    if( ! file_exists( $style_file_path ) ) {
        wp_mkdir_p( $style_file_path );
    }

    // Sanitize contents ( probably needs to be sanitized better, maybe with a CSS specific library )
    $contents = wp_filter_nohtml_kses( wp_strip_all_tags( $_POST['custom_css'] ) );

    // Replace the contents of the file with the new saved contents.
    $style_file = $style_file_path . '/my.css';
    file_put_contents( $style_file, $contents );

    // Redirect back to page with $_GET successes and errors
    wp_safe_redirect( add_query_arg( array(
        'success' => true,
    ), wp_get_referer() ) );
    exit();

// /* How to Enqueues the external CSS file */
add_action( 'wp_enqueue_scripts', 'my_external_styles' );
function my_external_styles() {
 
    wp_register_style( 'my-css', $style_file_path .'/my.css' );
    wp_enqueue_style( 'my-css' );
}
}