extension that inject tag to header without any additional config

I'm writing an extension for the community and I need it to inject an html tag to the head, which is not constant, eg:

<script>var something = "this value comes from php code";</script>

what's the best way to inject such a tag, to all site pages, without require the admin to do any additional config on the admin panel?

thanks!

EDIT:

Based on Bernhard Prange's answer, here's the solution for future reference (note - it just do alert don't actually inject any meaningful javascript, but you can easily put anything in the template):

app\design\frontend\base\default\template\page\html\inject_js.phtml

<script>alert("TEST")</script>

app\design\frontend\base\default\layout\test.xml

<?xml version="1.0" encoding="UTF-8"?>
<layout version="0.1.0">
    <default>
        <reference name="head">

                <block type="page/html" name="dynamicJs" as="dynamicJs" template="page/html/inject_js.phtml" />

        </reference>
    </default>
</layout>

app\code\community\SomeName\Test\etc\config.xml

  <!-- note, DON'T put this inside the <global> tag! -->
    <frontend>
      <layout>
        <updates>
          <test>
            <file>test.xml</file>
          </test>
        </updates>
      </layout>
    </frontend>