MySQL: Inserting Unique Column Value

Hi,
Scenario: I have a database table that stores blog posts. It looks like this:

CREATE TABLE `posts` (
`id` INT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY ,
`title` VARCHAR( 50 ) NOT NULL ,
`safe_title` VARCHAR( 60 ) NOT NULL ,
`content` TEXT NOT NULL ,
`user_id` INT UNSIGNED NOT NULL ,
UNIQUE (`safe_title`)
) ENGINE = MYISAM ;

The `safe_title` column contains the post title in a URL safe format. That is, the post title with all special characters removed. So a post title of "Hello World" becomes "hello_world". It's important that every post has a unique safe title, even if some posts have the same title. Post #1 with the title "Hello World" would have the safe title of "hello_world". Post #2 with the title "Hello World" would have the safe title "hello_world_2", and so on.

The problem: How to insert a unique safe title in only one or two db transactions? I don't want to create code like this:

$found = true;
$counter = 0;
do {
      ++$counter;
      $safe_title = getSafeTitle('Hello World') . '_' . $counter;
      $results = $db->execute("SELECT FROM `posts` WHERE `safe_title` = '$safe_title' LIMIT 1");
      if (count($results) == 0) {
            $found = false;
      }
} while ($found == true);
$db->insert("INSERT INTO ....") // Insert post with safe title

That little bit of code could loop through a dozen times before a unique title is found. Even worse, by the time $db->insert() is called, the safe title that I've found may no longer be unique (If the system is inserting thousands of new blog posts a minute).

So how do I avoid this problem?