Delete entry from related tables
There are several things you could try:

DELETE Style #1 : LEFT JOIN

DELETE e.*, m.* 
FROM `entries` e 
LEFT JOIN `magnets` m 
    ON m.`eid` = e.`id` 
WHERE e.`id` = %d


DELETE Style #2 : Two Independent DELETE queries
(without FOREIGN KEY defined)

DELETE FROM entries WHERE id = %d;
DELETE FROM magnets WHERE eid = %d;


DELETE Style #2b : Two DELETE queries : the order matters
 (when there is a FOREIGN KEY defined between the two tables
 with the ON DELETE NO ACTION property)

DELETE FROM magnets WHERE eid = %d;
DELETE FROM entries WHERE id = %d;


DELETE Style #3 : One DELETE query
 (when there is a FOREIGN KEY defined
 with the ON DELETE CASCADE property)

DELETE FROM entries WHERE id = %d;


Give it a Try !!!