This function is as simple as it sounds, it returns the name of the database your are currently connected to on the handle retrieved from a pg_connect() call.
<?php
    $pgsql_conn = pg_connect("dbname=mark host=localhost");
    if ($pgsql_conn) {
        print "Successfully connected to database: " . pg_dbname($pgsql_conn) .
         " on " .  pg_host($pgsql_conn) . "<br/>\n";
    } else {
        print pg_last_error($pgsql_conn);
        exit;
    }
?>
[mark@prodigy][*][~/public_html/php]>>>php pg_dbname.php
Content-type: text/html
X-Powered-By: PHP/4.3.1
Successfully connected to database: mark on localhost<br/>
Again this function is much more useful when you are changing which database you are connected to in your program.
Regards,  --mark