You don't indicate what version of Zen Cart you're using in your store, but older versions of ZC had more character-conversion issues that the more recent. In direct answer to your last question: Yes, a single character can nullify an entire PHP variable's data, if the character cannot be represented in the character-set your store is using.
A couple of other settings that might be pertinent:
- The DB_CHARSET value, present in your store's /includes/configure.php file.
- The CHARSET value, present in your store's /includes/languages/YOUR_TEMPLATE/english.php (if present) or /includes/languages/english.php (if the template-override version isn't there).
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