How php.ini limits connections to MySQL

php.ini contains two variables that you need to be aware of:

mysql.max_links = -1

mysql.max_persistent = -1

Setting these values to a number other than -1 (unlimited) in fact enforces a maximum number of connections PER php instance. For example:

If max_links is set to 3 and your script tries to establish more than 3 simultaneous links to various mysql databases, the script will throw a connection error after the 3rd link.

This can be a nasty one to diagnose when all else seems to be working.