(PHP, mysql) Copy column values to another column in the same table

I'm trying to copy title column to keywords column in database, so the keywords will be inserted automatically from the title.

http://ift.tt/1C2Pa2I

I want to add comma ', ' before each word for example.

" It's my first program "   

it will turn into

" It's, my, first, program, "

This the code I wrote.

<?php

  // $id =mysql_insert_id;
  $select_posts = mysql_query("SELECT * FROM `posts`");

  while($row = mysql_fetch_array($select_posts)){
        $id  = $row['post_id'];
        $text =  $row['post_title'];  

       $delim = ' \n\t,.!?:;';
       $tok = strtok($text, $delim);


    while ( $tok !== false){
          echo $tok1 = $tok.',';
          mysql_query("UPDATE `posts` SET  `post_keywords` =  '$tok1' WHERE `post_id` = $id  ");
          $tok = strtok($delim);
        }   
}

?>    

it insert the last word in each title column , because the words is overwritten by while loop.

Please help me .