How to create a customized field value by executing PHP code

This is an example of how to create a new field type that automatically creates a customized field value by executing PHP code

1. duplicate text field to create a new field type
   "text" field as type "myreference"
   How to Duplicate a Flexicontent Field to create a new Field Type
2. modify plugin function
   onBeforeSaveField(...)
   
   to be
   
   

   // Method to handle field's values before they are saved into the DB
   function onBeforeSaveField( &$field, &$post, &$file, &$item )
   {
   	// execute the code only if the field type match the plugin type
   	if ( !in_array($field->field_type, self::$field_types) ) return;
   	if ( !is_array($post) && !strlen($post) ) return;
   	
   	$post = array();
   	$post[0] = 'GMP'.$item->catid.'-'.$item->id;
   }

   
   
   
3. Create a field of type 'myreference' and set it to be hidden from for both frontend/backend item forms (the parameter is at the left when edit the field configuration)