Question

mysql_query("LOCK TABLES `counter` WRITE;") or die(mysql_error());
$res = mysql_query("SELECT `unID` FROM `counter`;") or die(mysql_error());
$row = mysql_fetch_assoc($res);
$unID = $row['unID'] + 1;
mysql_query("UPDATE `counter` SET `unID`=`unID`+1;") or die(mysql_error());
mysql_query("UNLOCK TABLES;") or die(mysql_error());

This is 100% guarantee that $unID will be unique for every user who visit web ?

mysql table type: MyISAM

Answer 1

From the manual:

Only the session that holds the lock can access the table. No other session can access it until the lock is released.

So the answer is yes, the number will be unique.