making a query based on the value of a textbox

i have a form and it uses the post method and i need to search for a username in a table...and the value of the textbox is the username..this code wont work..im having some troub, im fairly new to php can someone help me out...

<?php
echo "x_1 = " . $_POST['x_1'];
$db = mysql_connect("localhost", "root");
mysql_select_db("users",$db);
$result = mysql_query("SELECT * FROM info WHERE username = '$_POST['$x_1']' ",$db);

printf("ID: %s<br>\n", mysql_result($result,0,"id"));
printf("Username: %s<br>\n", mysql_result($result,0,"username"));
printf("Password: %s<br>\n", mysql_result($result,0,"password"));

?>

Try: