Description:
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The interactive shell (-a) should also work when the readline extension is compiled as shared object. This an be done by introducing a hook in the cli SAPI. An issue is that `php -h` can't give a proper indication about the availability as we'd have to startup PHP first. I assume we can keep the compile time decision there.

Test script:
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./configure --with-readline=shared
./sapi/cli/php -dextension=modules/readline.so -a

Expected result:
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php >