How to select nodes with field value equal to field value in current node using view?

Here's my recipe:

1. Create a view of content (block display)
2. Add a contextual filter for Content: nid
   1. in "When the filter value is NOT available" select "Content ID from URL"
   2. under "More" check the box for "Exclude"
3. Add a second contextual filter for your field event_ref
   1. in "When the filter value is NOT available" select "PHP code"
      1. Pull the field value off the current node, with something like the following:
         

         // If we're on a node page... 
         if (arg(0) == 'node' && is_numeric(arg(1)) && !arg(2)) { 
         
           // Load the node. 
           $node = node_load(arg(1));
         
           // Check the key of the field using devel module.
           // dpm($node->field_event_ref[$node->langcode][0]);
         
           // Get and return the field value. 
           $lang = $node->langcode;
           return $node->field_event_ref[$lang][0]['value']; 
         }

4. Save the view.
5. Add the block to the layout.

One thing to note is that if your event_ref field is not a text field (if it's a reference field or a taxonomy field, for example) that final key will be something other than 'value'.

You can enable the devel module and use the dpm() function to check!