PHP使用XML文件作为数据存储库

I have a simple PHP application, which uses MySQL DB, but I think that maybe the using of DB is needlessly for such easy operations.

Anyway, I hove some problems with the XML operations.

Let's say I want to have XML structure like this:

<root>
    <experiment>
       <name>test</name>
       <accessCount>5</accessCount>
       <downloadEntry>
           <date>2015-11-27</date>
           <comment>comment</comment>
       </downloadEntry>
       <downloadEntry>
           <date>2015-11-28</date>
           <comment>comment</comment>
       </downloadEntry>
    </experiment>
</root>

Now I would like to know, how to do these operations:

1. Count download entries (count of downloadEntry nodes) of experiment with name "test". Via XPATH?
2. Get download entries of experimetn with name test - but I would like to have pagination on this. So get download entries somehow like LIMIT 0,5.
3. The biggest problem is that, when there are no experiments - so the XML is , the loading of XML with simplexml_load_file fails. I can't open it. Yes, I can add the condition - if the XML is empty, donť open it. But I need to write to it and can't write if it isnť open.

Is there a solution for that?

Thanks everyone