Add note about inconsistent T_* constant values across PHP versions (thanks anonymous 11741)

git-svn-id: https://svn.php.net/repository/phpdoc/en/trunk@318883 c90b9560-bf6c-de11-be94-00142212c4b1

appendices/tokens.xml

@@ -13,6 +13,24 @@
   know that, here is a table with those identifiers, PHP-syntax and 
   references to the appropriate places in the manual.
  </para>
+ 
+ <note>
+  <title>Usage of T_* constants</title>
+  <para>
+   All tokens listed below are also defined as PHP constants. Their value is
+   automatically generated based on PHP's underlying parser infrastructure.
+   This means that the concrete value of a token may change between two PHP
+   versions. For example the <constant>T_FILE</constant> constant is
+   <literal>365</literal> in PHP 5.3, while the same value refers now to
+   <constant>T_TRAIT</constant> in PHP 5.4 and the value of <constant>T_FILE</constant>
+   is <literal>369</literal>. This means that your code should never rely directly
+   on the original T_* values taken from PHP version X.Y.Z, to provide some compatibility
+   across multiple PHP versions. Instead your code should utilize custom values
+   (using big numbers like <literal>10000</literal>) and an appropriate strategy that
+   will work with both PHP versions and T_* values.
+  </para>
+ </note> 
+ 
  <table>
   <title>Tokens</title>
   <tgroup cols="3">