You have to specify what to select.
$result = mysql_query("SELECT fieldname FROM table WHERE ID='$heelbericht'");
If you use * it will return all of the fields from the table.
Also if $heelbericht is not a numeric value you HAVE to put quotes around it. I would because with quotes is works either way. Also, make sure you use single quotes or escape "\" characters inside double quotes or it will break your string.
This is a no no:
$string = "SELECT * FROM table where name="$name"";
These are ok:
$string = "SELECT * FROM table where name='$name'";
$string = "SELECT * FROM table where name=\"$name\"";
Good Luck,
Rodney