CREATE TABLE staff
(
  id serial NOT NULL,
  f_name character varying(50),
  l_name character varying(50),
  CONSTRAINT staff_pkey PRIMARY KEY (id)
);

insert into staff(f_name,l_name) values ('Fons','Caffray');

CREATE OR REPLACE FUNCTION fn_get_something(
INOUT p_fname character varying)
RETURNS varchar(50)
AS
$BODY$
  select l_name from staff where f_name=$1;
$BODY$
LANGUAGE sql VOLATILE;

select fn_get_something('Fons') as lname;

/* result
lname
------------
Caffray
*/

try
{
    $dbh = new PDO($dsn, $user, $pass);
    $dbh->setAttribute(PDO::ATTR_ERRMODE, pdo::ERRMODE_EXCEPTION);
} catch (PDOException $e)
{
    print_r($e->getMessage());
    die();
}

try
{
    $fname = 'Fons';
    $sth = $dbh->prepare('select fn_get_something(?)');
    $sth->bindParam(1, $fname, PDO::PARAM_STR | PDO::PARAM_INPUT_OUTPUT, 50);
    $sth->execute();
    echo $fname;
} catch (PDOException $e)
{
    echo $e->getMessage();
}
/* result

Fons
*/