Sounds like you are using a old version of php maybe??
I don't think checking the code is the thing, as it worked with the previous host. And I can't sit and update a thousand files when I move my host. It's better to get the appropriate PHP version installed.
I think I'll have to contact the host. Hope its resolved.
I'm going to take a stab without seeing the PHP file and say you're moving from PHP4 to PHP5 or PHP4 to PHP5. Then in the class declaration you're declaring variables and using the method incompatible with the version you're on now.
It's really tough to say though without the versions you're using and a few lines around the error portion to know for sure. I might be completely wrong though I just think that's the most likely cause if none of the PHP code has changed and the files are intact and identical as they were at the previous host.
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diffent changes of php will give that error. I know when I updated to php 5.3 my board died on me. So I rolled back. Depending on the changes on the different php version, you should test it out before you move