Parsing XML with PHP, rewrite specific lines

I need to search and rewrite specific lines in xml file with PHP. I have this xml structure:

<contentList>
    <content contentuid="h0001d8b9g13d6g4605g85e9g708fe1e537c8">2p_some text.</content>
    <content contentuid="h000824a4gf6cfg4e7cgad12g3cf2b4473efe">some text.</content>
    <content contentuid="h000824a4gf6cfg4e7cgad12g3cf2b4473efe">some text.</content>
    <content contentuid="h0001d8b9g13d6g4605g85e9g708fe1e537c8">10txp_some text</content>
    <content contentuid="h000824a4gf6cfg4e7cgad12g3cf2b4473efe">15t_some text.</content>
</contentList>

I need to search lines with _ symbol inside the tags and rewrite numbers (2p, 4t...) with number of the line, but the letters after the number and _ must remain. Lines without _ symbol remain unchanged.

So it should look like this:

<contentList>
    <content contentuid="h0001d8b9g13d6g4605g85e9g708fe1e537c8">1p_some text.</content>
    <content contentuid="h000824a4gf6cfg4e7cgad12g3cf2b4473efe">some text.</content>
    <content contentuid="h000824a4gf6cfg4e7cgad12g3cf2b4473efe">some text.</content>
    <content contentuid="h0001d8b9g13d6g4605g85e9g708fe1e537c8">4txp_some text</content>
    <content contentuid="h000824a4gf6cfg4e7cgad12g3cf2b4473efe">5t_some text.</content>
</contentList>

It is possible? I'm really bad at XML parsing :D Any help will be appreciated. Thanks