you can use 'path' to specify the 'core'.
<?php
    $core = 'dictionary';
    $solr_server = array
    (
        'hostname'     => 'localhost',
        'port'         => '8983',
        'path'         => 'solr/' . $core,
    );
    $solr_client = new SolrClient($solr_server);
?>