if(in_array($request->get('post_type'), ['newQ', 'newP']))
{
    $category = implode(",", $request->get('catBox'));
}
else {
    $category = $request->get('catBox');
}

I recieved this error :

ErrorException in helpers.php line 747: preg_replace(): Parameter mismatch, pattern is a string while replacement is an array in helpers.php line 747 at HandleExceptions->handleError('2', 'preg_replace(): Parameter mismatch, pattern is a string while replacement is an array', 'D:\wamp\www\testSite\vendor\laravel\framework\src\Illuminate\Support\helpers.php', '747', array('search' => '?', 'replace' => array('test', array('appliances', 'Development'), 'Qtop', '1', '2016-06-21 07:46:34', '2016-06-21 07:46:34'), 'subject' => 'insert into posts (body, cat, post_type, user_id, updated_at, created_at) values (test, ?, ?, ?, ?, ?)', 'value' => array('appliances', 'Development')))

That is a query error. Seems you are trying to insert an array in your cat field.