compare two audio wav files by their voice

Not simple. Check out CMUSphinx and Julius projects:

• http://cmusphinx.sourceforge.net/
• http://julius.sourceforge.jp/en_index.php

Both are open source and provide a C API, Sphinx4 provides also a Java API. So if you want to make them work along with PHP you can create CGI scripts:

• http://httpd.apache.org/docs/current/howto/cgi.html

Or write a PHP extension:

• http://www.php.net/manual/en/internals2.structure.php
• http://devzone.zend.com/303/extension-writing-part-i-introduction-to-php-and-zend/

I addition to Cereal's recommendations, there were few voice (of course converted to audio) recognition applications written in PHP. I'm not sure if they still exist today.

1. Voice PHP
2. PHP Voice

Try searching these terms on sourceforge. I am pretty sure I forked this application some years ago, but due to lack of interest from the developers, the original author abandoned the project.

You will need FFMPEG PHP to use it as an audio comparator.