Issue with PHP variable

I'm not sure I undestand your question...

$music_choice = $_POST['music_choice'];
$qryupdate = "Update event_details set ".$music_choice."='$song_id' where user_id='".$_SESSION['oml_user_id']."'";
echo $qryupdate;

btw THIS IS NOT GOOD PROGRAMMING because you should escape your variables before insert them in a SQL query using function like http://php.net/manual/en/function.mysql-real-escape-string.php