A drop dead simple Habari plugin to load the highlight.js library.
This will highlight all code on the page marked up as <pre><code> .. </code></pre>.
If you are using HabariMarkdown plugin, you can simply use a code fence:
~~~
Your code here.
~~~
Additionally, you can apply a class to the <code class="php"> to help highlight.js know what language you are using (though I believe the script has ability to detect common languages and attempts to apply.)
Again, if using Markdown, to set the class it is as easy as
~~~php