kalau errornya seperti "supplied argument is not a valid MySQL result resource" pasti errornya karena salah sql query. Coba:
$dataalumni=mysql_query("SELECT t_a.id_alumni, t_a.nama_lengkap, t_a.alamat, t_a.no_tlp, t_a.tahun_lulus, t_a.foto, t_k.nama_instansi, tu.username
FROM alumni t_a
INNER JOIN tempat_kerja t_k ON
t_k.id_tempat = t_a.id_tempat_kerja INNER JOIN t_user tu ON tu.id_user = tu.id_user WHERE tahun_lulusan='$cari' or nama_lengkap LIKE '%$cari%'");