Learn basic PHP string syntax:
$sql = "DELETE FROM sv_info WHERE id='.$id.' LIMIT 1";
^--start of PHP string ^---end of PHP string
You're generating the literal query string
DELETE FROM sv_info WHERE id='.4.' LIMIT 1
Note how your bad attempt at PHP string concatenation actually became part of the query string. you're already IN a php string, so you can't execute PHP inside that string - PHP is not recursively embeddable/executable.
Either of these would work:
$sql = "DELETE FROM sv_info WHERE id='$id' LIMIT 1";
$sql = "DELETE FROM sv_info WHERE id='" . $id . "' LIMIT 1";
but of course, still leave you vulnerable to sql injection attacks.
