When I'm developing applications that would be used on client web serves of different configurations, often with shared hosting, which PHP 5 version should I assume most web servers have, e.g 5.2.x, 5.3.x, etc?
Version 5.2 is used by 84.9% of all the websites who use PHP version 5.
Version 5.3.3 is used by 39.7% of all the websites who use PHP version 5.3
You can see more statistical data from the following link:
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