that generally means you have compression enabled in php.ini by default so every file that is sent is compressed.
either comment out all lines to do with ob_start etc or disable the compression by default (this may be the better option)
to disable default use one of the two methods below:
.htaccess
Code:
php_flag zlib.output_compression Off
(php_flag may need to be renamed to PHP_value on some rare server setups)
php.ini
Code:
zlib.output_compression = Off