Newer version of PHP causing code incompatibilities

All of our pages which were previously running on a PSA 2.5 server which were done in php used the syntax <? echo "hello world"; ?>. In PSA 5 (or the different install of PHP) you must use the proper <?php "hello world"; ?> syntax. We are now on PHP Version 4.1.2 Our server crashed and we have installed newer software, now most of the PHP does not work because of the above difference in coding. Is there any easy way of allowing the old code to run without having to rewrite thousands of lines of code??? Or any suggestions would be appreciated Thanks in advance