I am currently using something like:
<?=($d === 'bar' || $d === 'foo') ? 'response' : null ?>
To create logic if $d is either 'bar' and 'foo', then return 'response' string. Is there a more elegant way to write this?
You have already written the elegant way. You can use the PHP in_array() function, The in_array() function searches an array for a specific value.
Note: If the search parameter is a string and the type parameter is set to TRUE, the search is case-sensitive.
<?=(in_array($d, ['foo', 'bar'])) ? 'response' : null ?>
You can make this code even better for readability
<?php $foo_bar = ["foo", "bar", "bla", "blaa"]; ?>
<?=(in_array($d, $foo_bar)) ? 'response' : null ?>