EDIT:
Ok for everyone who seems to be mentally disabled or something, check this out how it should be and why Halcyon's solution to swap haystack and needle DO MAKE SENSE:
$test = 'bla';
if(strpos($test, 'hauptkat') !== false OR strpos($test, 'kat') !== false)
{
echo 'hauptkat or kat is inside $test';
}
else
{
echo 'hauptkat or kat is NOT inside $test';
}