Question

Can someone explain why all this code works normally if PHP is only supposed to support a 256-character set?

I know that Content-Type tag interpret these characters if is on UTF-8.
But why PHP work it?

echo "匝";

if (preg_match('/啊/', "啊"))
    echo "Match";

if (preg_match('/\w/', "啊"))
    echo "Match";

Answer 1

Compare your code to:

if (preg_match('/^\w$/', "啊"))
    echo "Match";

regex /\w/ works because your multibyte char contains of 2 bytes: 0x53 and 0x1D. And first one, 0x53 looks like a valid single-byte char S

PS: this is valid way to match one multibyte letter:

var_dump(preg_match('/^\p{L}$/u', "匝", $matches));