How do I make this preg_match case insensitive?

Just add the i modifier after your delimiter (# in your case):

preg_match("#(.{100}$keywords.{100})#i", strip_tags($description), $matches);

If your delimiter is /, add an i after it :

preg_match("/your_regexp_here/i", $s, $matches); // i means case insensitive

If the i modifier is set, letters in the pattern match both upper and lower case letters.

Another option:

<?php
$n = preg_match('/(?i)we/', 'Wednesday');
echo $n;

https://php.net/regexp.reference.internal-options

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