There are no errors in the tests. Furthermore don't take the habit to post "issues" unless you are sure of your code: they are for problems in the kata not for problems in your code; you make the author loss honor... Moreover I don't think that the return value ist null, maybe -1. Good luck!

The description says:

return a sorted array r in lexicographical order of the strings of a1 which are substrings of strings of a2

In $this->revTest(inArray($a1, $a2), ["arp", "live", "strong"]); r is in lexical order, no?

In the PHP version the last test (visible) test case has an error:

 $a1 = ["live", "strong", "arp"];
 $this->revTest(inArray($a1, $a2), ["arp", "live", "strong"]);

As you can see it expects the array keys in a different order than they were given, which is different from what the first two test cases suggest.

I think it would be a good idea to add two more test-cases to the default ones. Namely the Cheshire cat and the one where 'shire' is surrounded by non-alphabetical characters.