mysql select that worked in php 4-mysql 3.25 but fails in php5-mysql 5

$sql = "
SELECT t1.id, t1.datum, t1.achternaam, t3.naam AS activiteit 
FROM ".PREFIX."inschrijvingen AS t1, ".PREFIX."kampen AS t2 
INNER JOIN ".PREFIX."activiteiten AS t3 
ON t3.id = t1.kampid AND t3.id = t2.kamp_id 
WHERE t1.verwijderd = '0' AND t1.oud = '0' AND t1.kampid = '".$kamp."' 
ORDER BY t1.datum DESC";

that worked on php4 and mysql 3.25 but fails on php 5 and mysql 5
I get the folloing error:

Unknown column 't1.kampid' in 'on clause'
but t1.kampid is an existing column

does any one know what this could be or how I could get the same results with another SELECT?

Dimmy