Migration to PHP5 causes corruption

I tried to upgrade from 2.8.6 to 2.9.1 but the process failed right at the end, requiring PHP5. We went back to 2.8.6. My provider migrated my site to a server running PHP5 and MySQL5. But many of the WordPress files were somehow corrupted in the process.

The corruption is similar to that which occurs moving a document from a Mac to a PC: extended ASCII characters get replaced by a small string of incorrect characters.

What can I do to prevent this from happening, or to correct it afterwards?

I can repeat the migration if necessary, as the original site still exists. But I don’t want to go through perhaps 200 files correcting the corrupted symbols.

Might it be caused by PHP5 parsing the files differently? Also, WordPress itself generally does a good job translating, for example, two hyphens or a hyphen surrounded by spaces into an em rule, or straight quotes into typographical quotes and the like. However, I usually enter these characters directly with keyboard codes. Does this, perhaps, upset PHP5? Because it’s these very characters which have become corrupted.

But anyway, can I prevent it when migrating to PHP5?

Thanks.