I repeated your steps and installed php5-mhash and then purged php5-mhash. In both cases php5-common was installed, respectively purged.
According to the man page of apt-get this seems to be intended behaviour:
purge
purge is identical to remove except that packages are removed and
purged (any configuration files are deleted too).
remove
remove is identical to install except that packages are removed
instead of installed....
According to your proposal, purge and remove would no longer reflect install. I am not sure if this is a good idea. I set this bug report to incomplete for now, reflecting that the issue has to be given more thought.
I repeated your steps and installed php5-mhash and then purged php5-mhash. In both cases php5-common was installed, respectively purged.
According to the man page of apt-get this seems to be intended behaviour:
purge
purge is identical to remove except that packages are removed and
purged (any configuration files are deleted too).
remove
remove is identical to install except that packages are removed
instead of installed....
According to your proposal, purge and remove would no longer reflect install. I am not sure if this is a good idea. I set this bug report to incomplete for now, reflecting that the issue has to be given more thought.