PHP 8.1 | MigrationGuide/New constants: add missing constants [4]

Refs:
* https://wiki.php.net/rfc/readonly_properties_v2
* php/php-src#7089
* php/php-src@6780aaa

Author: jrfnl

appendices/migration81/constants.xml

@@ -101,6 +101,16 @@
   </itemizedlist>
  </sect2>
 
+ <sect2 xml:id="migration81.constants.tokenizer">
+  <title>Tokenizer</title>
+
+  <itemizedlist>
+   <listitem>
+    <simpara><constant>T_READONLY</constant></simpara>
+   </listitem>
+  </itemizedlist>
+ </sect2>
+
 </sect1>
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