Clearify the array-do's-and-don'ts - thanks to Hugh W Prior (#17065)

git-svn-id: https://svn.php.net/repository/phpdoc/en/trunk@81397 c90b9560-bf6c-de11-be94-00142212c4b1

language/types.xml

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="iso-8859-1"?>
-<!-- $Revision: 1.76 $ -->
+<!-- $Revision: 1.77 $ -->
  <chapter id="language.types">
   <title>Types</title>
 
@@ -1204,7 +1204,9 @@ unset( $a[2] );
     <sect3 id="language.types.array.foo-bar">
      <title>Why is <literal>$foo[bar]</literal> wrong?</title>
      <para>
-      You might have seen the following syntax in old scripts:
+      You should always use quotes around an associative array index.
+      For example, use $foo['bar'] and not $foo[bar]. But why is $foo[bar]
+      wrong? You might have seen the following syntax in old scripts:
       <informalexample>
        <programlisting role="php">
 <![CDATA[
@@ -1214,8 +1216,14 @@ echo $foo[bar];
 ]]>
        </programlisting>
       </informalexample>
-      This is wrong, but it works. Then, why is it wrong? The reason is 
-      that, as stated in the <link linkend="language.types.array.syntax"
+      This is wrong, but it works. Then, why is it wrong? The reason is that
+      this code has an undefined constant (bar) rather than a string ('bar' -
+      notice the quotes), and PHP may in future define constants which,
+      unfortunately for your code, have the same name.  It works, because the
+      undefined constant gets converted to a string of the same name. 
+     </para>
+     <para>
+      As stated in the <link linkend="language.types.array.syntax"
       >syntax</link> section, there must be an expression between the 
       square brackets ('<literal>[</literal>' and '<literal>]</literal>').
       That means that you can write things like this: