title: Merging PHP changes with diff/patch
body: <p>I have three PHP files:</p>
  
  <ul>
  <li>1.php  - Version 1.0 of OSS software (vanilla)</li>
  <li>1a.php - Version 1.0 of OSS software (with customisations)</li>
  <li>2.php  - Version 1.1 of OSS software (vanilla)</li>
  </ul>
  
  <p>I would like to use diff/patch (or similar) to create a single patch file that will merge the modifications I made to version 1.0 with the upstream changes made in version 1.1. What is the best way to achieve this?</p>
  
  <p>Many thanks.</p>