PHP few functions from included file are not found while some are

<?php

echo $_SERVER['DOCUMENT_ROOT']."\web\home\security\security.php";

//including files
//--------------------------------------------------------------
include_once "home/database_handler.php";
require_once $_SERVER['DOCUMENT_ROOT']."\web\home\security\security.php";
include_once "home/session/session_handler.php";





//check session and assign if there ain't any nonce_key
//--------------------------------------------------------------
if (!session_check("nonce_key"))
{
    // generate the key
    $key=generate_rand_key("str");

    $set = set_to_session($key, "nonce_key");

    if ($set)
        echo "successfully set the session";
    else
        echo "session set failed!";
}





//check if any session is already set & POST supplied
//--------------------------------------------------------------
hasher("str", "salt");



?>

Here is a code fragment of reg.php

For security.php:

<?php

include_once("/../config/security_config.php");


// functions start here XD

//Generate a new key each time (to be used as nonce/rand key)
//------------------------------------------------------------------
function generate_rand_key($str) {
    $time = time();
    $key = md5($time);
    return $key;
}



function filter_user_input($input) {

    // INJECTION
    $input = mysql_real_escape_string($input);


    // XSS
    $input = htmlentities( htmlspecialchars($input));

    return $input;
}

function hasher($string, $salt) {
    // first md5 with salt
    $input = md5("$string.$salt");
    //$input = crypt($input);

    //using crypt function
    $input = crypt($input, $salt);

    return $input;


}



?>

And I get this error:

PHP Fatal error:  Uncaught Error: Call to undefined function generate_rand_key() in C:\inetpub\wwwroot\web\reg.php:20

It is interesting to note the function hasher doesn't receive this error even if I remove few lines above but the function generate_rand_key gets this error.

For require above, doesn't have any kind of error.

What's the problem here? Is it a bug?