Verify your host's version of PHP
Some Web hosts (e.g. 1and1.com) were a bit sloppy about upgrading php from 4.x to 5.x. The first step to troubleshooting this problem is to create a file called test.php that includes this snippet of code:
<? phpinfo(); ?>
Preview the results in a web browser. If the results returned are 4.x but the host claims to have installed 5.x, there's still hope.
1and1.com claims that their goal was to give developers the option of using either php 4 or php 5. By default, their Apache servers use php 4 for the .php extension.
They recommend changing all of your script extensions to .php5 if you prefer to use php 5. For most of us, that's not a good option.
Instead: create or modify your .htaccess file and place the following line AddType x-mapp-php5 .php in it.