To list active connections we do:
show processlist;
What to do to list old closed connections?
To list active connections we do:
show processlist;
What to do to list old closed connections?
The simple answer is that there's nothing provided by MySQL to list old connections. But you could capture the data flowing between your web front end and your database:
tcpdump -i eth0 -s 0 -w /tmp/mysql.cap tcp port 3306
run your PHP, have it fail, stop the capture and use Wireshark to see exactly what was going on.