PHP isset() with multiple parameters

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.

You just need:

if (!empty($_POST['search_term']) && !empty($_POST['postcode']))

isset && !empty is redundant.

The parameters of isset() should be separated by a comma sign (,) and not a dot sign (.). Your current code concatenates the variables into a single parameter, instead of passing them as separate parameters.

So the original code evaluates the variables as a unified string value:

isset($_POST['search_term'] . $_POST['postcode']) // Incorrect

While the correct form evaluates them separately as variables:

isset($_POST['search_term'], $_POST['postcode']) // Correct