You still have a space in the query string.
Try to keep your query strings in the format of:
var1=value&var2=value ....
and urlencode() your variables:
echo "<font face=\"Tahoma\" size=\"2\"><a href=\"display.php?mid=" . urlencode($a_row["username"]) . "\">" . urlencode($a_row["username"]) . "</a></font>";
You also missing the > on the opening <a> tag: '<a href.... </a>