php 5 and mysql failure

I'm using a php script which performs three xml queries to other three servers to retrieve a set of ids and after I do a query to mysql of the kind SELECT * FROM table WHERE id IN ('set of ids'); Although I'm sure the connection to the database is ok, I sometimes get an error of this kind: *Warning*: mysql_fetch_object(): supplied argument is not a valid MySQL result resource in ... This does not happen every time i run the script, only sometimes. If I echo the query, copy and paste in phpmyadmin, or if I perform the same query in a script that does only the query without the rest it works! After troubleshooting this issue I noticed that it usually failed when I had a big set of ids (positive response from more than one server). This means that the script used a bigger amount of memory and probably more resources, but I did not get an "out of memory error", I got the one described bfore. My question is, is there any kind of limit somewhere in php5 or in mysql? Thanks for help, YEHUDI GARRETT