Is == in PHP a case-sensitive string comparison?

Yes, but it does a comparison byte-by-byte.

If you're comparing unicode strings, you may wish to normalize them first. See the Normalizer class.

Example (output in UTF-8):

$s1 = mb_convert_encoding("\x00\xe9", "UTF-8", "UTF-16BE");
$s2 = mb_convert_encoding("\x00\x65\x03\x01", "UTF-8", "UTF-16BE");
//look the same:
echo $s1, "\n";
echo $s2, "\n";
var_dump($s1 == $s2); //false
var_dump(Normalizer::normalize($s1) == Normalizer::normalize($s2)); //true

Yes, == is case sensitive.

You can use strcasecmp for case insensitive comparison

Yes, == is case sensitive.

Incidentally, for a non case sensitive compare, use strcasecmp:

<?php
    $var1 = "Hello";
    $var2 = "hello";
    echo (strcasecmp($var1, $var2) == 0); // TRUE;
?>