I wanted to point out something that is not obvious from the documentation here or in any google searches that I've done. Everything after the 'pgsql:' is passed to the PQconnectdb function of the pgsql library as a connection string. This means 2 important key things:
1) 'username' is not a valid option of the PQconnectdb connection string. Use 'user' instead (ie 'user=<username>' instead of 'username=<username>').
2) You can utilize ANY option of the PQconnectdb function in this way. For those trying to figure out how to enable ssl connections through the pgsql PDO connection process, like me, as per Postgresql standards, the 'sslmode' parameter can be set via the DSN. So, for example, 'sslmode=require' will require an ssl connection. Look at the documentation for the pgsql library version you are using for all options of both the sslmode parameter and any other parameters that are available.