PHP，MySQL选择所有if

I have problem with PHP and MySQL please help..

$lokalita_s = $_POST['lokalita_s'];

$query = "SELECT nazov, lokalita FROM reality WHERE lokalita = '".$lokalita_s."' ORDER BY id";

............ But if ($lokalita_s == "nezáleží")... then i want to select every thing from database.. something like this :

$query = "SELECT nazov, lokalita FROM reality ORDER BY id";

............

This is not working :

$lokalita_s = 0;
$lokalita_s = NULL;
$lokalita_s = *;

I really dont want to use it like if else.. because i want to use more variables in that query and it won't be effective

Try

$lokalita_s = $_POST['lokalita_s'];
$wherClause = null;

if($lokalita_s != "nezáleží") {
   $wherClause = "WHERE lokalita = '" . $lokalita_s . "'";
}

$query = "SELECT nazov, lokalita FROM reality $wherClause ORDER BY id";

Something along these lines? (This works in Oracle)

$lokalita_s = $_POST['lokalita_s'];

$query = "
SELECT nazov, lokalita FROM reality WHERE lokalita = '". $lokalita_s."'
UNION 
select nazov,lokalita from reality where '". $lokalita_s. "' = 'nezáleží'
order by id
"