if ($name=$subject=$contens) {
        $condition="name like '%$keyword%' or subject like '%$keyword%' or contens like '%$keyword%'";
} else if ($name=$subject) {
        $condition="name like '%$keyword%' or subject like '%$keyword%'";
} else if ($name=$contens) {
        $condition="name like '%$keyword%' or contens like '%$keyword%'";
} else if ($subject=$contens) {
        $condition="subject like '%$keyword%'+ '+ ' or contens like '%$keyword%'";
} else if ($name==on) {
        $condition="name like '%$keyword%'";
} else if ($subject==on) {
        $condition="subject like '%$keyword%'";
} else if ($contens==on) {
        $condition="contens like '%$keyword%'";
}

아까부터 봐도 오타는 아닌거 같은데..
if문 제외하고 첫번째 else if문하고 4, 5번쨰 else if 문이 자꾸 오류가 나서여..
혹시 이렇게 쓰면 안되는건가여?