<?php
// Functions
function db_connect($host,$db,$user,$pass){
  if(@mysql_connect($host, $user, $pass)){
    @mysql_select_db($db)or
    $content = sql_debug();
  }
  else {
    $content = sql_debug();
  }
  return @$content;
}

// Variables to change
$sql_username = "";
$sql_password = "";
$sql_host = "";
$sql_db = "";

// What is your post type that you want to limit this change to?
// IE: Only posts from 'exhibitors' will have this change
// $post_type = 'post_type';

// What is the meta source? In this example it is website_link
// this is the "old" value you want to copy to the new value
$old_value = '_mds_';

// What is the meta destination? In this example lets use tf_ex_site_url
// BOTH key names must already exists in this instance of the script. If
// both don't exist connected with the post_id, the value wont be copied.
$new_value = '_yoast_wpseo_metadesc';

// Ok Go
db_connect($sql_host,$sql_db,$sql_username,$sql_password);
$counter = 0;
// Applies to everything.
// OLD // $query = "SELECT * from wp_posts WHERE post_type = '$post_type'";
$query = "SELECT * from wp_posts";
$result = mysql_query($query);
while ($row = mysql_fetch_array($result)) {
  $post_id = $row['ID'];
  $query2 = "SELECT * from wp_postmeta WHERE post_id = '$post_id'";
  $result2 = mysql_query($query2);
  while ($row2 = mysql_fetch_array($result2)) {
    // Do update here
    if ($row2['meta_key'] == $old_value) {
      $counter++;
      $temp_new = $row2['meta_value'];
      $query_update = "UPDATE wp_postmeta SET meta_value = '$temp_new' WHERE meta_key = '$new_value' AND post_id = '$post_id'";
      $result_update = mysql_query($query_update);
      // Un-comment the below line if you want to BLANK the old value (ie in this case make website_link = to nothing after
      // updating the value of tf_exhibitor_sort
      // mysql_query("UPDATE wp_postmeta SET meta_value = '' WHERE meta_key = '$old_value' AND post_id = '$post_id'");
    }
  }
}

echo $counter." records updated.";

?>