Add warning about not abusing the return value of PDO::exec.

git-svn-id: https://svn.php.net/repository/phpdoc/en/trunk@187114 c90b9560-bf6c-de11-be94-00142212c4b1

reference/pdo/functions/PDO-exec.xml

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="iso-8859-1"?>
-<!-- $Revision: 1.3 $ -->
+<!-- $Revision: 1.4 $ -->
 <!-- Generated by xml_proto.php v2.2. Found in /scripts directory of phpdoc. -->
 <refentry id="function.PDO-exec">
  <refnamediv>
@@ -54,6 +54,25 @@
    or deleted by the SQL statement you issued. If no rows were affected, 
    <function>PDO::exec</function> returns <literal>0</literal>. 
   </para>
+
+  <warning>
+   <para>
+    Do not rely on the return value as an indicator of success. The following
+    example incorrectly relies on the return value of
+    <function>PDO::exec</function>:
+    <programlisting role="php">
+<![CDATA[
+<?php
+$db->exec() or die($db->errorInfo());
+?>
+]]>
+    </programlisting>
+    The previous example is incorrect because the SQL statement may
+    legitimately return 0 affected rows, and <function>die</function> does
+    not distinguish between <literal>0</literal> and &false;.
+   </para>
+  </warning>
+
  </refsect1>
 
  <refsect1 role="examples">