Choosing the correct SQL query based off user input - php

Question

Lets say the user only entered in an artist to search by, how do I get PHP to recognize that the user is only looking for an artist and should therefore setup a query only for the artist. The way I have currently done it works, but it is very ineffective when the user wants a variety of search options.
$artist = $_POST["artistSearch"];
$topic= $_POST["topicSearch"];
$date = $_POST["dateSearch"];
$title = $_POST["titleSearch"];
$artistLength = strlen($artist);
$topicLength = strlen($topic);
$dateLength = strlen($date);
$titleLength = strlen($title);
if ($artistLength>2 && $topicLength<2 && $dateLength<2 && $titleLength<2) {
$sql=("Select * FROM music WHERE artist = '$artist' ORDER BY date");
}
if ($artistLength>2 && $topicLength>2 && $dateLength<2 && $titleLength<2) {
$sql=("Select * FROM music WHERE artist = '$artist' AND topic = '$topic' ORDER BY date");
}
if ($artistLength>2 && $topicLength>2 && $dateLength>2 && $titleLength<2) {
$sql=("Select * FROM music WHERE artist = '$artist' AND topic = '$topic' AND date = '$date' ORDER BY date");
}
if ($artistLength>2 && $topicLength>2 && $dateLength>2 && $titleLength>2) {
$sql=("Select * FROM music WHERE artist = '$artist' AND topic = '$topic' AND date = '$date' AND title = '$title' ORDER BY date");
}
$result = mysqli_query($con,$sql);

Answer 1

Simple, you build up the query string in stages. Ignoring your sql injection attack vulnerability, and assuming that all options should be ANDed together, you can do something like:
$options = array();
if ($artist) {
$options[] = "artist = '$artist'";
}
if ($topic) {
$options[] = "topic = '$topic'";
}
etc...
$where_clause = implode(' AND ', $options);
$sql = "SELECT ... WHERE $where_clause";