how to validate custom field in contact form

[henro]

hi all,
i ask again 

i want to validate contact form, where the field i want to validate is by custom field,

i have readed this
http://wiki.civicrm.org/confluence/display/CRMDOC/hook_civicrm_validateForm

its very help,
but the example field validated there is by " addres " field and as i know it is not a custom field .

this the warning i face

 
    Notice: Undefined index: custom_9 in myextension_civicrm_validateForm() (line 119 of C:\xampp\htdocs\drupal\extentions\com.example.myextension\myextension.php).
    Warning: Invalid argument supplied for foreach() in myextension_civicrm_validateForm() (line 119 of C:\xampp\htdocs\drupal\extentions\com.example.myextension\myextension.php).




could anyone give to me an example to validate form conctac by custom field

thanks in advance

[joanne]

 AS the board is called General Discussion (please no support requests here!)      this question does not belong here.

Probably the developer's board will would be a better place for it.

[henro]

thanks for the info Joanne