Mysql function that query a table and reconstruct a string from the query-result

I'm a newbie in mysql programming, and i would create a mysql function myFunction with a string parametre; that query a table and reconstruct a string from the query-result like this example :

myTable
 ---------------
|id   |  value   |
 ---------------
| id1 |value1    |
| id2 |value2    |
| id3 |value3    |
| id4 |value4    |
 ---------------

Calling this function is like this

myFunction('value2#value1#value4')

and must return

 'id2#id1#id4'

Thank you very much