Example of an EE extension hook to replace an {exp:channel:entries} custom field variable.

ext.hook_example.php

<?php
/*
        Replace any template tags as needed
    */
    function channel_entries_tagdata($tagdata, $row, &$channel)
    {    
        // has this hook already been called?
        if (isset($this->EE->extensions->last_call) && $this->EE->extensions->last_call)
        {
            $tagdata = $this->EE->extensions->last_call;
        }
        
        $values = array();
        $params = array();

        foreach ($this->EE->TMPL->tagparams as $param => $value) 
        {
            $values[] = is_array($value) ? implode('',$value) : $value; 
            $params[] = is_array($param) ? implode('',$param) : $param;
        }
        
        $key = 'uhura:tagdata::'. $this->site_id .':'. $row['entry_id'] .':'. implode(':', $params) .'|'. implode(':', $values) .':'. $this->lang_id .':'. $this->EE->uhura_helper->_get_display_status();

        if($title = $this->EE->uhura_helper->memcache_get($key, 'entry'))
        {
            // No further processing, use $tagdata below...
        }
        else
        {
            // Only switching {title} for now
            $query = $this->EE->db->select('title')
                                  ->where('lang_id', $this->lang_id)
                                  ->where('entry_id', $row['entry_id'])
                                  ->where('site_id', $this->EE->config->item('site_id'))
                                  ->where('status', $this->EE->uhura_helper->_get_display_status())
                                  ->get('uhura_data');
                               
            if($query->num_rows() == 0 AND $this->setting['display_fallback'])
            {
                $query = $this->EE->db->select('title')
                                      ->where('entry_id', $row['entry_id'])
                                      ->where('site_id', $this->EE->config->item('site_id'))
                                      ->get('channel_titles');
                                  
                $title = $query->row('title');
            }
            else
            {
                $title = $query->row('title');
            }
            
            // Prevents array to string conversions. Was getting $row data back without entry_ids, no idea why.
            $title = is_array($title) ? '' : $title;
            
            $this->EE->uhura_helper->memcache_save($key, 'entry', $title);
        }
        
        $tagdata = $this->EE->TMPL->swap_var_single('title', $title, $tagdata); 
        
        return $tagdata;
    }