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Get to parent folder in codeigniter
require_once("../dompdf_config.inc.php");

//APPPATH gives you path till application/ folder
require_once(APPPATH.'your_DOMpdf_file_path');

$this->config->load('filename');

// Stored in an array with this prototype: $this->config['blog_settings'] = $config
    $this->config->load('blog_settings', TRUE);

$this->config->load('blog_settings', FALSE, TRUE);