PHP - Help With Unit Testing

I’ve got a colleague who was brought in as a testing specialist to help improve code quality and reduce bugs. I’ve done a little testing before, but am no means an expert.

When they joined I was really excited to learn from them as testing is definitely an area I lack experience.

He’s got good coverage and he writes tests for literally everything he does, but it’s something like 95% mocks as opposed to what I would call “black box” testing; testing the input and output. Does that sound reasonable?

As I say, I’m not an expert, but his testing strategy doesn’t seem particularly effective and it puts constraints over exactly how code is implemented rather than testing that things behave correctly.

I need to create the unique SKU (Stock Keeping Unit) name for each products mixed with
product.id,product.name,product.category and product.brand. There is no problem to do it,

i.e.: SKU for a product with

id=13,
name=Bio Clean green and
category=INSECTS KILLERS
brand=HARPIC

becomes : BI-IKHA0013

This is how I tried:

$inm = "Bio Clean green500ml";
$cnm = "INSECTS KILLERS";
$bnm = "HARPIC ";

echo SKU_gen($inm, $cnm,$bnm,20).'<br>';


function SKU_gen($pname, $cat=null, $brand=null, $id = null, $l = 2){
    $results = ''; // empty string

    $str1 = array_shift(explode(' ',$pname));
    $str1 = strtoupper(substr($str1, 0, $l));

    $str2 = array_shift(explode(' ',$cat));
    $str2 = strtoupper(substr($str2, 0, $l));

    $str3 = array_shift(explode(' ',$brand));
    $str3 = strtoupper(substr($str3, 0, $l));

    $id = str_pad($id , 4, 0, STR_PAD_LEFT);

    $results .= "{$str1}-{$str2}{$str3}{$id}";
    return $results;
}

But this function is not working as expected when category of brand values become NULL.