There isn’t (and shouldn’t) be an answer with any specific version for this. New versions come out, old versions get discontinued.
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At the minimum you should use supported version of PHP. This ensures it still receives bug fixes and (or for less recent version) security updates.
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Preferably you should be use the latest stable version of PHP. This ensures you get best possible performance.
PHP site has handy Supported Versions page, which clearly covers currently relevant versions and roadmap for their support.