I could be wrong here correct me if I am wrong. I tried to fetch the URL in the code with the variable replaced as in the code. i.e. (zipcode=60642 and unit=F) http://xml.weather.yahoo.com/forecastrss?p=60642&u=F
I got following

<rss version="2.0" xmlns:yweather="http://xml.weather.yahoo.com/ns/rss
+/1.0" xmlns:geo="http://www.w3.org/2003/01/geo/wgs84_pos#"> 
<channel> 
<title>Yahoo! Weather - Error</title> 
<description>Yahoo! Weather Error</description> 
<item>
<title>City not found</title>
<description> Invalid Input /forecastrss?p=60642&amp;u=F</description>
</item>
</channel>
</rss><!-- api2.weather.ac4.yahoo.com compressed/chunked Sun Jul 19 22
+:10:00 PDT 2009 -->
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I tied the same for other zipcode (60641) and got following

<rss version="2.0" xmlns:yweather="http://xml.weather.yahoo.com/ns/rss
+/1.0" xmlns:geo="http://www.w3.org/2003/01/geo/wgs84_pos#"> 
<channel>     
<title>Yahoo! Weather - Chicago, IL</title> 
<link>http://us.rd.yahoo.com/dailynews/rss/weather/Chicago__IL/*http:/
+/weather.yahoo.com/forecast/USIL0225_f.html</link> 
<description>Yahoo! Weather for Chicago, IL</description> 
<language>en-us</language> 
<lastBuildDate>Sun, 19 Jul 2009 11:51 pm CDT</lastBuildDate> 
<ttl>60</ttl> 
<yweather:location city="Chicago" region="IL" country="US"/> 
<yweather:units temperature="F" distance="mi" pressure="in" speed="mph
+"/> 
<yweather:wind chill="64" direction="0" speed="0" /> 
<yweather:atmosphere humidity="70" visibility="10" pressure="30.13" ri
+sing="0" /> 
<yweather:astronomy sunrise="5:32 am" sunset="8:21 pm"/> 
<image> 
<title>Yahoo! Weather</title> 
<width>142</width> 
<height>18</height> 
<link>http://weather.yahoo.com</link> 
<url>http://l.yimg.com/a/i/us/nws/th/main_142b.gif</url> 
</image> 
<item> 
<title>Conditions for Chicago, IL at 11:51 pm CDT</title> 
<geo:lat>41.84</geo:lat> 
<geo:long>-87.68</geo:long> 
<link>http://us.rd.yahoo.com/dailynews/rss/weather/Chicago__IL/*http:/
+/weather.yahoo.com/forecast/USIL0225_f.html</link> 
<pubDate>Sun, 19 Jul 2009 11:51 pm CDT</pubDate> 
<yweather:condition text="Partly Cloudy" code="29" temp="64" date="Sun
+, 19 Jul 2009 11:51 pm CDT" /> 
<description><![CDATA[ 
><img src="http://l.yimg.com/a/i/us/we/52/29.gif"/><br /> 

<b>Current Conditions:</b><br /> 
Partly Cloudy, 64 F<BR /> 
<BR /><b>Forecast:</b><BR /> 
Sun - Partly Cloudy. High: 71 Low: 54<br /> 
Mon - Sunny. High: 78 Low: 62<br /> 
<br /> 
<a href="http://us.rd.yahoo.com/dailynews/rss/weather/Chicago__IL/*htt
+p://weather.yahoo.com/forecast/USIL0225_f.html">Full Forecast at Yaho
+o! Weather</a><BR/><BR/> 

(provided by <a href="http://www.weather.com" >The Weather Channel</a>
+)<br/> 

]]></description> 
<yweather:forecast day="Sun" date="19 Jul 2009" low="54" high="71" tex
+t="Partly Cloudy" code="29" /> 
<yweather:forecast day="Mon" date="20 Jul 2009" low="62" high="78" tex
+t="Sunny" code="32" /> 
<guid isPermaLink="false">USIL0225_2009_07_19_23_51_CDT</guid> 
</item> 
</channel> 
</rss><!-- api2.weather.ac4.yahoo.com compressed/chunked Sun Jul 19 22
+:30:42 PDT 2009 -->
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If am not missing anything here the possible reason for the error is that yahoo is not returning any data for zip=60642 and there is no code to handle this condition therefore "$nodeset->get_node($index);" returns a undef value.
I tried the same with "http://weather.yahooapis.com/forecastrss" and got the same results (no data for 60642), therefore I don't think changing URL will help. What you need is to handle the condition in the code.