Properly handle whitespace in PHP examples

PhD no longer renders NBSPs, but regular spaces instead[1].  To retain
the desired indentation, we set the CSS propery `white-space:pre`.
However, we cannot do that for the `.phpcode` class directly, because
`highlight_string()` emits additional line breaks at the top and the
bottom of the examples, so we use a more specific selector to avoid
this "padding".

[1] <87f794bafe>

styles/theme-base.css

@@ -1324,6 +1324,10 @@ div.tip p:first-child {
     font-weight: normal;
 }
 
+.phpcode code span span {
+    white-space: pre;
+}
+
 /* }}} */