Shift Timing of xml captions (subtitle) using php

I'm using xml captions in my project and I need to use php to shift forward (+ sec) or shift back (- sec)

This is example of captions, I need to shift it (begin and end) 1.5 seconds forward

<div xml:id="captions">
    <p begin="00:01:53.591" end="00:01:56.299">Look down, look down.</p>
    <p begin="00:01:57.117" end="00:02:00.106">Don't look them in the eye.</p>
</div>

Result should be like this:

<div xml:id="captions">
    <p begin="00:01:55,091" end="00:01:57,799">Look down, look down.</p>
    <p begin="00:01:58,617" end="00:02:01,606">Don't look them in the eye.</p>
</div>

How do I get those results noted just above?