How to view query error in PDO PHP

try {
	$db = new PDO("mysql:host=".HOST.";dbname=".DB, USER, PW);
	$st = $db->prepare("SELECT * FROM c6ode");
}
catch (PDOException $e){
	echo $e->getMessage();
}

How can I check the mysql error for the query in above case?

You need to set the error mode attribute PDO::ATTR_ERRMODE to PDO::ERRMODE_EXCEPTION.
And since you expect the exception to be thrown by the prepare() method you should disable the PDO::ATTR_EMULATE_PREPARES* feature. Otherwise the MySQL server doesn't "see" the statement until it's executed.

<?php
try {
	$pdo = new PDO('mysql:host=localhost;dbname=test;charset=utf8', 'localonly', 'localonly');
	$pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
	$pdo->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);
	
	
	$pdo->prepare('INSERT INTO DoesNotExist (x) VALUES (?)');
}
catch(Exception $e) {
	echo 'Exception -> ';
	var_dump($e->getMessage());
}

prints (in my case)

Exception -> string(91) "SQLSTATE[42S02]: Base table or view not found: 
1146 Table 'test.doesnotexist' doesn't exist"

see http://wezfurlong.org/blog/2006/apr/using-pdo-mysql/
EMULATE_PREPARES=true seems to be the default setting for the pdo_mysql driver right now. The query cache thing has been fixed/change since then and with the mysqlnd driver I hadn't problems with EMULATE_PREPARES=false (though I'm only a php hobbyist, don't take my word on it...)

*) and then there's PDO::MYSQL_ATTR_DIRECT_QUERY - I must admit that I don't understand the interaction of those two attributes (yet?), so I set them both, like

$pdo = new PDO('mysql:host=localhost;dbname=test;charset=utf8', 'localonly', 'localonly', array(
	PDO::ATTR_EMULATE_PREPARES=>false,
	PDO::MYSQL_ATTR_DIRECT_QUERY=>false,
	PDO::ATTR_ERRMODE=>PDO::ERRMODE_EXCEPTION
));

I'm using this without any additional settings:

if (!$st->execute()) {
    print_r($st->errorInfo());
}

I'm guessing that your complaint is that the exception is not firing. PDO is most likely configured to not throw exceptions. Enable them with this:

$db->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION); 

a quick way to see your errors whilst testing:

$error= $st->errorInfo();
echo $error[2];

/* Provoke an error -- the BONES table does not exist */

$sth = $dbh->prepare('SELECT skull FROM bones');
$sth->execute();

echo "\nPDOStatement::errorInfo():\n";
$arr = $sth->errorInfo();
print_r($arr);

output

Array
(
    [0] => 42S02
    [1] => -204
    [2] => [IBM][CLI Driver][DB2/LINUX] SQL0204N  "DANIELS.BONES" is an undefined name.  SQLSTATE=42704
)