random_compat.php

<?php

if (!function_exists('random_bytes')) {
    function random_bytes($size)
    {
        if(!is_int($size))
            throw new Exception('random_bytes: $size must be an int');
        if($size <= 0)
            throw new Exception('random_bytes: $size must be positive');
        
        if(function_exists('mcrypt_create_iv'))
        {
            $result = mcrypt_create_iv($size, MCRYPT_DEV_URANDOM);
        }
        else if(function_exists('openssl_random_pseudo_bytes'))
        {
            $result = openssl_random_pseudo_bytes($size, $isSecure);
            if($isSecure !== true)
            {
                throw new Exception("random_bytes: openssl_random_pseudo_bytes returned insecure data");
            }
        }
        else
        {
            throw new Exception("random_bytes: No RNG found"); 
        }
        
        if(!is_string($result) || (strlen($result) !== $size))
        {
           throw new Exception("random_bytes: RNG is unavailable or broken");
        }
        return $result;
    }
}
 
if (!function_exists('random_int')) {
    function random_int($min, $max)
    {
        if(version_compare(PHP_VERSION, '5.1.0') < 0)
            trigger_error("random_int: This version of PHP is not supported", E_USER_ERROR);
        if(!is_int($min))
            throw new Exception('random_int: $min must be an int');
        if(!is_int($max))
            throw new Exception('random_int: $max must be an int');
        if($min > $max)
            throw new Exception('random_int: $min must be less or equal to $max');
            
        $range = $max - $min + 1;
        // the rejection probability is at most 0.5, so this corresponds to a failure probability of 2^-128 for a working RNG
        for($attempts = 0; $attempts < 128; $attempts++)
        {
            // generate a random integer
            $bytes = random_bytes(PHP_INT_SIZE);
            $value = 0;
            for($i = 0; $i < PHP_INT_SIZE; $i++)
            {
                $value = ($value << 8) | ord($bytes[$i]);
            }
            
            if(!is_int($range))
            {
                if(($value >= $min) && ($value <= $max))
                {
                    return $value;
                }
            } else {
                $value &= PHP_INT_MAX;
                // equivalent to (PHP_INT_MAX + 1) % range, but avoids int overflows
                // I'm assuming PHP_INT_MAX + 1 is a power-of-two and that integer's are represented as two's complement.
                $reject = (-$range & PHP_INT_MAX) % $range;
                if($value >= $reject)
                {
                    return ($value % $range) + $min;
                }
            }
        }
        throw new Exception("random_int: RNG is broken - too many rejections");
    }
}

random_string.php

<?php

if (!function_exists('random_string')) {
    function random_string($length, $charset = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789")
    {
        if(!is_int($length))
            throw new Exception('random_string: $length must be an integer');
        if($length < 1)
            throw new Exception('random_string: $length must be positive');
        if(!is_string($charset))
            throw new Exception('random_string: $charset must be a string');
        if(strlen($charset) < 1)
            throw new Exception('random_string: $charset must contain at least one element');
        
        $randomString = "";
        for ($i = 0; $i < $length; $i++)
            $randomString .= $charset[random_int(0, strlen($charset) - 1)];
    
        return $randomString;
    }
}