Adding custom functions PHP to use in a views template I want to use a custom function in views-view-fields--field-name.tpl.php template file. Where do I have to put it? Is it possible to use it in template.php in some preprocess functions?
You can use a preprocess function in your template.php file. For example:

function YOURTHEME_preprocess_views_view_fields__field_name(&amp;$vars) {
  // Do your changes for the field here
}