Question on the port PHP uses to connect to remote MySQL

Here is my problem. I have PHP running on one machine, mysql on another one. Everything works fine till I put a firewall on the PHP machine, which blocks pretty much every port there is except port 80. Now, PHP can't connect to the mysql database anymore. If I disable the firewall on the PHP machine it works again. So I eliminate the reasons down to that PHP must uses some port for mysql return the query. The question is which port does PHP use for the connection? I'm NOT talking about 3306 which is used by Mysql. Anyone could help? Thanks in advance. Louis