<?php

$key_size = 256;  //For a 256 bit key, you can use 128 or 512 as well. 

$key = uuid_gen(($key_size / 8)); //8 bits in a byte

​

echo 'key length is ' .strlen($key) ."\n"; //The number of bytes

​

/* Returns Raw Binary */

function uuid_gen($length) {

​

    $fp = @fopen('/dev/urandom','rb');

​

    if ($fp !== FALSE) {

        $result .= @fread($fp, $length);

        @fclose($fp);

    } else {

        trigger_error('Can not open /dev/urandom.');

    }

​

    return $result;

}