Since you're specifying ARRAY_FORMAT as the input format, it's expecting the columns to be like this:
$input = array(array('col1', 'val1'),
array('col2', 'val2'),
array('col3', 'val3'),
array('col4', 'ladhania'));
The alternate formats example may also be useful.
Is there a php client to search cassandra using solr?
DataStax Enterprise has integrated Solr support, which you can query through either the Solr API, CQL (in recent versions), or the Thrift API (e.g. phpcassa).