Mysql user-defined function

[Programmer Programmer]

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