Clarify the output of example code about undefined variable usage (#2679

)

Accessing undefined variables generate an E_WARNING since PHP 8 (and will result in error exceptions being thrown in PHP 9).

Co-authored-by: George Peter Banyard <[email protected]>

language/variables.xml

@@ -265,7 +265,12 @@ test();
    </informalexample>
 
    <simpara>
-    This script will not produce any output because the echo statement
+    This script will generate an undefined variable <constant>E_WARNING</constant>
+    (or a <constant>E_NOTICE</constant> prior to PHP 8.0.0)
+    diagnostic. However, if the
+    <link linkend="ini.display-errors">display_errors</link> INI setting is set to hide
+    such diagnostics then nothing at all will be outputted.
+    This is because the echo statement
     refers to a local version of the <varname>$a</varname> variable,
     and it has not been assigned a value within this scope.  You may
     notice that this is a little bit different from the C language in