runkit
The runkit extension provides means to modify constants, user-defined functions, and user-defined classes. It also provides for custom superglobal variables and embeddable sub-interpreters via sandboxing.
This package is meant as a feature added replacement for the classkit package. When compiled with the --enable-runkit=classkit option to ./configure, it will export classkit compatible function definitions and constants.
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You can also view the list of all php extensions on this page.