Clarify what types of values can be given to declare directives.

Fixes doc bug #68177 (declare() + named constant = "Cannot convert to ordinal
value").


git-svn-id: https://svn.php.net/repository/phpdoc/en/trunk@335128 c90b9560-bf6c-de11-be94-00142212c4b1

language/control-structures/declare.xml

@@ -36,6 +36,25 @@ declare (directive)
    The encoding directive was added in PHP 5.3.0
   </simpara>
  </note>
+ <para>
+  As directives are handled as the file is being compiled, only literals may
+  be given as directive values. Variables and constants cannot be used. To
+  illustrate:
+  <informalexample>
+   <programlisting role="php">
+<![CDATA[
+<?php
+// This is valid:
+declare(ticks=1);
+
+// This is invalid:
+const TICK_VALUE = 1;
+declare(ticks=TICK_VALUE);
+?>
+]]>
+   </programlisting>
+  </informalexample>
+ </para>
  <para>
   The <literal>statement</literal> part of the
   <literal>declare</literal> block will be executed - how