WordPress at a glance
function is not described

random_bytes() WP 1.0

Unless open_basedir is enabled, use /dev/urandom for random numbers in accordance with best practices

Why we use /dev/urandom and not /dev/random

No Hooks.

Return

String.

Usage

random_bytes( $bytes );
$bytes(int) (required)

Code of random bytes: wp-includes/random_compat/random_bytes_dev_urandom.php VER 5.0.3

<?php
function random_bytes($bytes)
{
    static $fp = null;
    /**
     * This block should only be run once
     */
    if (empty($fp)) {
        /**
         * We use /dev/urandom if it is a char device.
         * We never fall back to /dev/random
         */
        $fp = fopen('/dev/urandom', 'rb');
        if (!empty($fp)) {
            $st = fstat($fp);
            if (($st['mode'] & 0170000) !== 020000) {
                fclose($fp);
                $fp = false;
            }
        }

        if (!empty($fp)) {
            /**
             * stream_set_read_buffer() does not exist in HHVM
             * 
             * If we don't set the stream's read buffer to 0, PHP will
             * internally buffer 8192 bytes, which can waste entropy
             * 
             * stream_set_read_buffer returns 0 on success
             */
            if (function_exists('stream_set_read_buffer')) {
                stream_set_read_buffer($fp, RANDOM_COMPAT_READ_BUFFER);
            }
            if (function_exists('stream_set_chunk_size')) {
                stream_set_chunk_size($fp, RANDOM_COMPAT_READ_BUFFER);
            }
        }
    }

    try {
        $bytes = RandomCompat_intval($bytes);
    } catch (TypeError $ex) {
        throw new TypeError(
            'random_bytes(): $bytes must be an integer'
        );
    }

    if ($bytes < 1) {
        throw new Error(
            'Length must be greater than 0'
        );
    }

    /**
     * This if() block only runs if we managed to open a file handle
     * 
     * It does not belong in an else {} block, because the above 
     * if (empty($fp)) line is logic that should only be run once per
     * page load.
     */
    if (!empty($fp)) {
        $remaining = $bytes;
        $buf = '';

        /**
         * We use fread() in a loop to protect against partial reads
         */
        do {
            $read = fread($fp, $remaining); 
            if ($read === false) {
                /**
                 * We cannot safely read from the file. Exit the
                 * do-while loop and trigger the exception condition
                 */
                $buf = false;
                break;
            }
            /**
             * Decrease the number of bytes returned from remaining
             */
            $remaining -= RandomCompat_strlen($read);
            $buf .= $read;
        } while ($remaining > 0);
        
        /**
         * Is our result valid?
         */
        if ($buf !== false) {
            if (RandomCompat_strlen($buf) === $bytes) {
                /**
                 * Return our random entropy buffer here:
                 */
                return $buf;
            }
        }
    }

    /**
     * If we reach here, PHP has failed us.
     */
    throw new Exception(
        'Error reading from source device'
    );
}