How to create variable length UTF-8 string in PHP?

I am looking to create a variable length UTF-8 string in PHP. I tried this method initially:

utf8_encode(mcrypt_create_iv(256, MCRYPT_DEV_URANDOM))

But it gave me a string longer than 256 chars

I then tried this method:

md5(mcrypt_create_iv(32, MCRYPT_DEV_URANDOM))

Which works for a 32 character long string but if I want a 256 long string I need to concatenate it 8 times which seems inefficient.

How would I do this efficiently in PHP? Thanks