Description:
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it is possible to add into php any callback function which handles spam?

Reproduce code:
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var_dump(filter_var($_REQUEST['message'], FILTER_CALLBACK, 'antispam'));


function antismap($message) {
  return exec('spamassasin $message');
}

// i can't use exec because of safe mode.
// so it whould be nice to have any
// option to embeded antispam function

Expected result:
----------------
set_ini (antispam, '/usr/bin/spamassasin');
var_dump(filter_var($_REQUEST['message'], FILTER_ANTISPAM));