The problem is that data:image/png;base64,
is included in the encoded contents. This will result in invalid image data when the base64 function decodes it. Remove that data in the function before decoding the string, like so.
function base64_to_jpeg($base64_string, $output_file) {
// open the output file for writing
$ifp = fopen( $output_file, 'wb' );
// split the string on commas
// $data[ 0 ] == "data:image/png;base64"
// $data[ 1 ] ==
$data = explode( ',', $base64_string );
// we could add validation here with ensuring count( $data ) > 1
fwrite( $ifp, base64_decode( $data[ 1 ] ) );
// clean up the file resource
fclose( $ifp );
return $output_file;
}