Ok I found one problem. A function is passing an object, but it's not being passed correctly on the PHP 5 server.
$worksheet = new Spreadsheet_Excel_Writer_Worksheet($this->_BIFF_version,
$name, $index,
$this->_activesheet, $this->_firstsheet,
$this->_str_total, $this->_str_unique,
$this->_str_table, $this->_url_format,
$this->_parser);
function Spreadsheet_Excel_Writer_Worksheet($BIFF_version, $name,
$index, &$activesheet,
&$firstsheet, &$str_total,
&$str_unique, &$str_table,
&$url_format, &$parser)
When I echo or print_r the $url_format or the $parser variables in PHP 4 it shows the object, but in PHP 5 it is not showing the object. The variable is actually blank... thoughts?