The parameter(s) to isset() must be a variable reference and not an expression (in your case a concatenation); but you can group multiple conditions together like this:
if (isset($_POST['search_term'], $_POST['postcode'])) {
}
This will return true only if all arguments to isset() are set and do not contain null.
Note that isset($var) and isset($var) == true have the same effect, so the latter is somewhat redundant.
Update
The second part of your expression uses empty() like this:
empty ($_POST['search_term'] . $_POST['postcode']) == false
This is wrong for the same reasons as above. In fact, you don't need empty() here, because by that time you would have already checked whether the variables are set, so you can shortcut the complete expression like so:
isset($_POST['search_term'], $_POST['postcode']) &&
$_POST['search_term'] &&
$_POST['postcode']
Or using an equivalent expression:
!empty($_POST['search_term']) && !empty($_POST['postcode'])
Final thoughts
You should consider using filter functions to manage the inputs:
$data = filter_input_array(INPUT_POST, array(
'search_term' => array(
'filter' => FILTER_UNSAFE_RAW,
'flags' => FILTER_NULL_ON_FAILURE,
),
'postcode' => array(
'filter' => FILTER_UNSAFE_RAW,
'flags' => FILTER_NULL_ON_FAILURE,
),
));
if ($data === null || in_array(null, $data, true)) {
// some fields are missing or their values didn't pass the filter
die("You did something naughty");
}
// $data['search_term'] and $data['postcode'] contains the fields you want
Btw, you can customize your filters to check for various parts of the submitted values.