Código PHP:
$criterio = "SELECT * FROM bd.contratos WHERE cif = '$id' AND ";
$criterio .= "(idcontrato LIKE '%$txt_criterio%' OR cif LIKE '%$txt_criterio%' OR idservicio LIKE '%$txt_criterio%' OR fAlta LIKE '%$txt_criterio%' OR fBaja LIKE '%$txt_criterio%' OR Horas LIKE '%$txt_criterio%')";