1. all of my servers run PHP5/MySQL5; all run threads perfectly.
2. You'll find that some older scripts don't work as is; most providers will offer ways to run their scripts with php5 (usually they rely on register globals being on, wheras in PHP5 they're off by default).
3. threads will run perfectly on PHP5
4. 'eh i'll always be around for any threads questions hehe; most of the others will be too
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5. Yeh, went from PHP4.3.10 to PHP5.2 on several machines; 0 problems from my upgrades.