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<source lang="php">
echo "foo";
</source>

converts to:

<div dir="ltr" class="mw-geshi mw-code mw-content-ltr" typeof="mw:Extension/source" data-parsoid="{...}" about="#mwt3">
<div class="php source-php">
<pre class="de1"> 
</pre></div>
</div><span about="#mwt3" data-parsoid="{}">

</span>
<p data-parsoid="{...}">echo "foo";
&lt;/source&gt;</p>

The paragraph at the end is not tagged as #mwt3 and so becomes editable.