form validation and checking for allowed values for example in listbox

[eluser]seba22[/eluser]
[quote author="CroNiX" date="1354475386"]Show your form element that this rule is being applied on. Something is strange with your mail test. It says "invalid string" where it should be showing the value of the form field.[/quote]

Sorry, i just enter this as a string
To just express, that string should not pass validation because simple "aaa" actually should pass.

Here's part form

Code:

<input type="text" name="admin_title" value="<?php echo $populate['admin_title']; ?>">  
    <?php echo form_error('admin_title'); ?>

Sorry, for make if confusing

Added:
I remove close php tag, still ISE error