$pdo = new PDO('mysql:dbname=db_test;host=localhost', 'root', '123456');

PHP Fatal error:  Uncaught PDOException: SQLSTATE[HY000] [2002] No such file or directory in test.php:2
Stack trace:
#0 test.php(2): PDO->__construct('mysql:dbname=db...', 'root', '123456')
#1 {main}
  thrown in test.php on line 2

$pdo = new PDO('mysql:dbname=db_test;host=127.0.0.1', 'root', '123456');