Me neither, but it's actually mentioned in the description:

Not all strings will contain digits. In this case and the case for any non-strings, return -1.

Although it's incosistent with the instruction:

Given a string of characters,

just echo gettype().
If it ain't a string:

In this case and the case for any non-strings, return -1.

Kata description isn't clear. I wasn't expecting an array as an input.

Thank You!

I have actully changed PHP version in my IDE settings, but I haven't changed my php config file. Nevertheless, it would be nice if newer PHP versions were supported.

Not a kata issue, it's like that in PHP 7.0

The following code doesn't work properly:
$s[-1]. I can't access string character by using negative index. I have checked it on my PHPStorm, on old PHP versions (5.4, 7.0) and it works correctly.

Description is already updated.

Wrong you can pass it with System.out.print and System.out.printf as long as you print a line as the description says and not just the phrase hello world ;)

System.out.print won't pass the test, it has to be System.out.println. I think both options are viable.

It would be nice to add some info in the description, that we should round up our result, when it has some decimal places.