I downloaded+installed the PHP example the SDK provides at http://www.byteshift.de/YahooSearchExample.php; It's limited to 10 results per query (and 1000 queries daily, exactly as Google's API), so IMO it's useless except for playing around – crippled intentionally...!

Often screen-scraping is faster & gives more daily hits – how ironic. Then again, if you want to be ToS-compliant...

Phil Ringnalda says 5,000 queries per day. I will have to check myself soon...
http://philringnalda.com/blog/2005/02/but_the_yahoo_api_gives_me_5000_queries_a_day.php

"At last at last at last, some competition for the Google API, with its puny limit of a thousand queries a day. The spanking-new Yahoo API gives you 5000 a day, the SDK includes BSD-licensed examples in Perl, Python, PHP, Java and JavaScript (Google's SDK includes Java and .NET examples), and accesses Image, Web, Video, Local, and News searches (last time I looked, Google did Web, Cache, and Spelling). Oh, and Yahoo's API uses REST rather than SOAP, though I don't want to go there."