Added note about variable naming.

git-svn-id: https://svn.php.net/repository/phpdoc/en/trunk@22177 c90b9560-bf6c-de11-be94-00142212c4b1

language/variables.xml

@@ -4,17 +4,40 @@
   <sect1 id="language.variables.basics">
    <title>Basics</title>
 
-   <para>
+   <simpara>
     Variables in PHP are represented by a dollar sign followed by the
     name of the variable. The variable name is case-sensitive.
+   </simpara>
+
+   <para>
+    Variable names follow the same rules as other labels in PHP. A
+    valid variable name starts with a letter or underscore, followed
+    by any number of letters, numbers, or underscores. As a regular
+    expression, it would be expressed thus:
+    '[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*'
+   </para>
+   
+   <note>
+    <simpara>
+     For our purposes here, a letter is a-z, A-Z, and the ASCII
+     characters from 127 through 255 (0x7f-0xff).
+    </simpara>
+   </note>
+
+   <para>
     <informalexample>
      <programlisting role="php"> 
 $var = "Bob";
 $Var = "Joe";
-echo "$var, $Var"; // outputs "Bob, Joe"
+echo "$var, $Var";      // outputs "Bob, Joe"
+
+$4site = 'not yet';     // invalid; starts with a number
+$_4site = 'not yet';    // valid; starts with an underscore
+$täyte = 'mansikka';    // valid; 'ä' is ASCII 228.
      </programlisting>
     </informalexample>
    </para>
+
    <para>
     In PHP3, variables are always assigned by value. That is to say,
     when you assign an expression to a variable, the entire value of