PHP problem in the mysql query

[Slartibartfast]

Hi everyone, first time here and I hope you can help me. I'm just starting with php and I have a problem. I get an error back from my browser when I load a page, which worked perfectly before,:

"Database query failed: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '' at line 1"

When adding this bit of code:

    function get_subject_by_id($subject_id) {
        global $connection;
        $query = "SELECT * ";
        $query .= "FROM subjects ";
        $query .= "WHERE id = {$subject_id}";
        //$query .= "LIMIT 1";
        $subject_set = mysql_query($query, $connection);
        confirm_query($subject_set);
        
        //if no rows are returned, then fetch_array will return false
        if ($subject = mysql_fetch_array($subject_set)) { 
            return $subject;
        }else{
            return NULL;
        }
    } 

I've been over it and over it but I can't figure out what I'm doing wrong. Any help would be apreciated.