[MySql] problema con ricerca like

[techno]

Salve a tutti sto avendo un problema con una ricerca LIKE e il carattere jolly "_"

$search=trim($_POST["search"]);
mysql_query("SELECT IDUser, Company, FirstName, LastName, DateRegister, Activaction FROM register WHERE Company LIKE '$search_' OR FirstName LIKE '$search_' OR FirstName LIKE '$search_' OR LastName LIKE '$search_' ORDER BY Company Asc"
... 

mi da errore come se $search_ fosse una variabbile:
Notice: Undefined variable: search_