Document dangling else handling

Cf. <https://bugs.php.net/51016#1619439579>.

language/control-structures/else.xml

@@ -37,6 +37,32 @@ if ($a > $b) {
   linkend="control-structures.elseif">elseif</link>).
 
  </para>
+ <note>
+  <title>Dangling else</title>
+  <para>
+   In case of nested <literal>if</literal>-<literal>else</literal> statements,
+   an <literal>else</literal> is always associated with the nearest <literal>if</literal>.
+   <informalexample>
+    <programlisting role="php">
+<![CDATA[
+<?php
+$a = false;
+$b = true;
+if ($a)
+    if ($b)
+        echo "b";
+else
+    echo "c";
+?>
+]]>
+    </programlisting>
+   </informalexample>
+   Despite the indentation (which does not matter for PHP), the <literal>else</literal>
+   is associated with the <literal>if ($b)</literal>, so the example does not produce
+   any output. While relying on this behavior is valid, it is recommended to avoid
+   it by using curly braces to resolve potential ambuigities.
+  </para>
+ </note>
 </sect1>
 
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