On yahoo hosting, there is no public_html. Everything is already in yahoo's public_html section.
On common cPanel-configured hosting (such as Bluehost), you have a home directory, in which you also have a public_html folder (and a www alias which points to public_html).
Inside your public_html folder you'll find all the files that can be accessed via a browser when visiting your site.
When your account was set up, it probably already contained an index.html file, and/or several others. If you didn't delete those before uploading your own files, then it's very possible that those are taking precedence (ie: the index.html is loading before the index.php gets a chance to be found). In that case, delete the index.html file.
If that doesn't solve it, you'll need to contact their tech support again to figure out what exactly you're doing wrong.
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