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What is meta data

Meta data is data about data
Usually the meta data describes how the data is composed in terms of names and types. It may describe constraints as well.

In a strong typed language souce code contains data about parameters, fields, variables and function return values. In a weak typed language like php this metadata is not present in the source. With PhpPeanuts meta data is explicitly added using meta objects: classDescriptors and propertyDescriptors.