str_replace函数

$tag = mysql_real_escape_string($_GET['tag']);
$tag = str_replace("-", " ", $_GET['tag']);

$query = mysql_query("SELECT * FROM tags WHERE name = '$tag'");
$row = mysql_fetch_assoc($query);

if (!mysql_num_rows($query)) exit('That tag dont exist.');

I have a tag with spaces in its name, how should i do to replace the spaces with hyphens? I think the above code should work but it doesnt :/

Update:

$tag = str_replace(' ', '-', $_GET['tag']);
$tag = mysql_real_escape_string($tag);

$query = mysql_query("SELECT * FROM tags WHERE name = '$tag'");
$row = mysql_fetch_assoc($query);

if (!mysql_num_rows($query)) exit('That tag dont exist');

Db tags

`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`name` varchar(40) COLLATE utf8_unicode_ci DEFAULT NULL



INSERT INTO `tags` (`id`, `name`) VALUES
(1, 'test'),
(2, 'test test'),
(3, 'test test test test');

(3, 'test test test test');

$tag = mysql_real_escape_string($_GET['tag']);
$query = "SELECT * FROM tags WHERE name = '$tag'";
$result = mysql_query($query) or die(mysql_error());
if ( false===($row=mysql_fetch_assoc($result)) ) exit('No such tag.');

Other way around:

$tag = str_replace(' ', '-', $_GET['tag']);
$tag = mysql_real_escape_string($tag);

(When you see str_replace(A, B, C), think "replace A with B in C")

(Also, escaping the string should happen after everything else. And you accidentally used $_GET['tag'] instead of $tag your second time.)