UPDATE shop_tshirts AS T1, shop_tshirts AS T2
SET T1.categorie = T2.categorie
WHERE T1.article_id = T2.article_id
AND T1.design_id = T2.design_id
AND T1.shop_id = 429942
AND T2.shop_id = 266497
"Update the categorie of all items with a shop_id of 429942 to match the categorie of items with shop_id of 266497 when the article_id and design_id also match."
No PHP needed. Do it in any MySQL query tool.