如何启用php以使用postgresql？

<?php

try {
   $dbh = new PDO('pgsql:host=localhost;port=5432;dbname=###;user=###;password=##');
   echo "PDO connection object created";
}
catch(PDOException $e)
{
      echo $e->getMessage();
}

?>
<?php

try {
   $dbh = new PDO('pgsql:host=loca