Index: wp-includes/wp-db.php
===================================================================
--- wp-includes/wp-db.php	(revision 28006)
+++ wp-includes/wp-db.php	(working copy)
@@ -1338,6 +1338,15 @@
 				}
 			}
 
+			// Persistent connections with mysqli are prepended with 'p:', supported on PHP5.3+
+			if ( ! $new_link ) {
+				if ( version_compare( phpversion(), '5.3.0', '>=' ) ) {
+					$host = "p:$host";
+				} else {
+					_doing_it_wrong( 'wpdb::db_connect', __( 'Disabling MYSQL_NEW_LINK with MySQLi requires PHP 5.3 or later.' ), '3.9.1' );
+				}
+			}
+
 			if ( WP_DEBUG ) {
 				mysqli_real_connect( $this->dbh, $host, $this->dbuser, $this->dbpassword, null, $port, $socket, $client_flags );
 			} else {