Im also suffered from this problem & simply, by adding port number after the ip address saved me.
$dsn = 'mysql:dbname=sms_messenger;host=127.0.0.1:8889'; $user = 'root'; $password = 'root';
This works for Mac OS
If you don't know your mysql port number, then
You can easily find the port number on MAMP home page
OR
Type following command while running the MAMP server to switch the terminal into mysql
OMBP:mamp Omal$ /Applications/MAMP/Library/bin/mysql --host=localhost -uroot -proot
Then type
mysql> SHOW GLOBAL VARIABLES LIKE 'PORT';