Case-insensitive constants are deprecated as of PHP 7.3.0

While there is already a respective changelog entry, we also
add a warning to the respective parameter.

git-svn-id: http://svn.php.net/repository/phpdoc/en@345952 c90b9560-bf6c-de11-be94-00142212c4b1

Author: cmb

trunk/reference/misc/functions/define.xml

@@ -57,6 +57,11 @@
        <literal>CONSTANT</literal> and <literal>Constant</literal> represent
        different values.
       </para>
+      <warning>
+       <simpara>
+        Defining case-insensitive constants is deprecated as of PHP 7.3.0.
+       </simpara>
+      </warning>
       <note>
        <para>
         Case-insensitive constants are stored as lower-case.