The connection string is wrong for the example :
$db = new PDO("ibm:DRIVER={IBM DB2 ODBC DRIVER};DATABASE=testdb;" .
"HOSTNAME=11.22.33.444;PORT=56789;PROTOCOL=TCPIP;", "testuser", "tespass");
You get the PDO Exception with the error code -1329.
You have to write :
$db = new PDO(
"ibm:DRIVER={IBM DB2 ODBC DRIVER};DATABASE=testdb;" .
"HOSTNAME=11.22.33.444;PORT=56789;PROTOCOL=TCPIP;UID=testuser;PWD=testpass",
"",
"");