Getting a PHP PDO connection from a mysql_connect()?

Both extensions internally use EG(persistent_list) to store the persistent connection handle. But they create different hashes/keys for this list, so they can't find entries of the respective other extension.

The mysql extension creates keys of the form "mysql_<host&port>_<user>..." while pdo builds "PDO:DBH:DSN=<dsn>:<user>:....". The hashes are used almost like array-keys in a php script. (Over-)simplyfied example:

function pconnect($host,$user,$pass) {
  global $persistent_list;
  $hashkey = sprintf("extensionname_%s_%s_%s", $host, $user, $pass);
  if ( isset($persistent_list[$hashkey]) ) {
    // use stored connection
  }
  else {
    // create new connection
  }
}

So the answer is: No, the connections will not be shared between and re-used by the mysql extension and PDO.

If you are using two different APIs (i.e. mysql_* and PDO), PHP will generate two different connections.

And, as a "proof", consider this portion of code :

$db = mysql_connect('localhost', 'USER', 'PASSWORD');
$pdo = new PDO('mysql://@localhost/astralblog', 'USER', 'PASSWORD');
sleep(5);

Running this will cause two distinct connections, on the MySQL server -- which will sleep for 5 seconds :

mysql> show processlist;
+----+------------+-----------------+------------+---------+------+-------+------------------+
| Id | User       | Host            | db         | Command | Time | State | Info             |
+----+------------+-----------------+------------+---------+------+-------+------------------+
| 41 | astralblog | localhost:46551 | astralblog | Sleep   |  188 |       | NULL             |
| 42 | astralblog | localhost:46552 | astralblog | Sleep   |  188 |       | NULL             |
| 43 | astralblog | localhost       | astralblog | Query   |    0 | NULL  | show processlist |
| 64 | astralblog | localhost       | NULL       | Sleep   |    4 |       | NULL             |
| 65 | astralblog | localhost       | NULL       | Sleep   |    4 |       | NULL             |
+----+------------+-----------------+------------+---------+------+-------+------------------+
5 rows in set (0,00 sec)

(The connections in question are the two last one, which appeared when I started the PHP script, and disappeared after 5 seconds)