Maybe remove some of those single quotes...
mysql_query ("UPDATE members SET memberCounter =memberCounter + 1 WHERE memberID = '$session_user'");
Try this with phpMyAdmin:
Pick a member that exists in the database table. Note the members id and the memberCount.
Create the sql query and paste it in the sql query tab. Then execute it.
Look back at the memberCount for that member. Did it get increased by one?
If it worked, your query statement is ok and the problem lies in your PHP code.
If it didn't work, then the query is incorrect or possibly you have a faulty schema declaration. Check the memberCount and members id types. Also check to see if you have any potential normalization issues.
If the problem lies in your PHP code, then do this:
Always create an $sql variable and then create the string on the fly. Never create SQL statements inside the mySQL_query() function. Why? Because you won't be able to echo it to debug the string before execution. I always do this in my code so that during development, I can check to see if I am forming the right strings. More times than not, that is where I screw up and can isolate the problem immediately.
That is:
$sql = "blah blah blah";
echo $sql; // check it here!
mySql_query($sql);
rather than:
mySql_query("blah blah blah");
// how am I going to check the string inside the function?!
