Description:
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The docs state "If you are connecting to a database defined in tnsnames.ora, append the name of the database to the DSN prefix and your DSN is complete."

However on php 5.1.2 i've had to append 'dbname=' first to get it to work. 

E.g. (my database name is 'prod'):
<?php
// should work according to docs but fails
$dbh = new PDO('oci:prod', 'user', 'pass'); /* fails with ORA-12560: TNS:protocol adapter error */

// This works:
$dbh = new PDO('oci:dbname=prod', 'user', 'pass');
?>