Could not generate 6 digit random number including date time using PHP

I need one help . I need to generate random number including date and time using PHP. I am explaining my code below.

$random=generateRandom();
echo $random;
function generateRandom() {
    $result = base_convert((float) rand() / (float) getrandmax() * round(microtime(true) * 1000),6, 36);
    return $result;
}

The above function giving the 3 digit output. Here i need to generate up to 6 digit. Please help me.