Document that heredoc labels are the same as other PHP labels.

git-svn-id: https://svn.php.net/repository/phpdoc/en/trunk@32411 c90b9560-bf6c-de11-be94-00142212c4b1

language/types.xml

@@ -169,11 +169,14 @@ $a = 1.234; $a = 1.2e3;
        </row>
       </tbody>
      </tgroup>
-    </table></para>
+    </table>
+   </para>
+
    <para>
     You can escape any other character, but a warning will be issued
     at the highest warning level.
    </para>
+
    <para>
     The second way to delimit a string uses the single-quote ("'")
     character. When a string is enclosed in single quotes, the only
@@ -182,13 +185,19 @@ $a = 1.234; $a = 1.2e3;
     a single-quoted string. Variables will <emphasis>not</emphasis> be
     expanded inside a single-quoted string.
    </para>
+
    <simpara>
     Another way to delimit strings is by using here doc syntax
     ("&lt;&lt;&lt;").  One should provide an identifier after
     <literal>&lt;&lt;&lt;</literal>, then the string, and then the
     same identifier to close the quotation. The closing identifier
     <emphasis>must</emphasis> begin in the first column of the line.
+    The label used must follow the same naming rules as any other
+    label in PHP: it must contain only alphanumeric characters and
+    underscores, and must start with a non-digit character or
+    underscore.
    </simpara>
+
    <para>
     Here doc text behaves just like a double-quoted string, without
     the double-quotes. This means that you do not need to escape quotes
@@ -229,6 +238,7 @@ EOT;
      </programlisting>
     </example>
    </para>
+
    <note>
     <para>
      Here doc support was added in PHP 4.