Please pay great attention to the $length parameter! It is exactly the return string length, NOT the length of raw binary hash result.

I had a big problem about this --
I thought that hash_pbkdf2(...false) should equals to bin2hex(hash_pbkdf2(...true)) just like md5($x) equals bin2hex(md5($x, true)). However I was wrong:

hash_pbkdf2('sha256', '123456', 'abc', 10000, 50, false); // returns string(50) "584bc5b41005169f1fa15177edb78d75f9846afc466a4bae05"
hash_pbkdf2('sha256', '123456', 'abc', 10000, 50, true); // returns string(50) "XKŴ��Qw�u��j�FjK���BFW�YpG    �mp.g2�`;N�"
bin2hex(hash_pbkdf2('sha256', '123456', 'abc', 10000, 50, true)); // returns string(100) "584bc5b41005169f1fa15177edb78d75f9846afc466a4bae05119c82424657c81b5970471f098a6d702e6732b7603b194efe"

So I add such a note. Hope it will help someone else like me.