function base64_to_jpeg( $base64_string, $output_file ) {
    $ifp = fopen( $output_file, "wb" );
    fwrite( $ifp, base64_decode( $base64_string) );
    fclose( $ifp );
    return( $output_file );
}

$image = base64_to_jpeg( $my_base64_string, 'tmp.jpg' );