By whitelisting the entries you do expect.
<?php
$post = array(
'parent_id' => 1,
'type' => 'foo',
'title' => 'bar',
'body' => 'foo bar',
'tags' => 'foo, bar',
'one' => 'foo',
'two' => 'bar',
'three' => 'qux'
);
$whitelist = array(
'parent_id',
'type',
'title',
'body',
'tags'
);
$filtered = array_intersect_key( $post, array_flip( $whitelist ) );
var_dump( $filtered );
Anyway, using Cassandra as a data-store is of course not a reason not to do validation on the data you’re receiving.