MySql: Function to insert entry and return id... if already exists, return id of existing?

[eluser]inktri[/eluser]
What's an efficient algorithm for a function that tries to insert an entry... if the entry already exists, it returns the id of that already existing entry, else it inserts the new entry and returns the id of the new entry?

thanks for the help

[eluser]inktri[/eluser]
possible to do better than something like:?

Code:

<?php
$query = mysql_query("SELECT entryID FROM table WHERE {condition}");

//If it does exist
if (mysql_num_rows($query) > 0){
   $row = mysql_fetch_assoc($query);
   echo "Entry already exists, the id is {$row['entryID']}";

//if it doesn't exist
} else {
  
   //insert entry to DB
   $insert = mysql_query("INSERT INTO table (...) VALUES (...)");
   $id = mysql_insert_id();
  
   echo "New entry added, the id of the new entry is $id";
}

?>