phpunit thinks two equal strings are not equal

I'm using some code to convert ISO-8859 string to utf8. Then I want to assert, that it has been converted correctly. The code is:

class EncodingTest extends TestCase {

    public function testImapUtf8()
    {
        $pairs = [
            ['=?ISO-8859-13?Q?Darba_s=E2k=F0ana_ar_Gmail?=', 'Darba sākšana ar Gmail'],
            ['Normal text', 'Normal text'], 
            ['Darba sākšana ar Gmail', 'Darba sākšana ar Gmail'],
        ];

        foreach ($pairs as $pair) {
            $this->assertEquals(($pair[1]), (imap_utf8($pair[0])));
        }
    }
}

However, for the first pair, it gives me an error:

1) EncodingTest::testImapUtf8
Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
-'Darba sākšana ar Gmail'
+'Darba sākšana ar Gmail'

The strings look the same. I guest there are some invisible characters. how do I test that imap_utf8 works as expected?