function genRandomString() 
{
    $length = 8;
    $characters = '0123456789abcdefghijklmnopqrstuvwxyz';
    $string = '';    

    for ($p = 0; $p < $length; $p++) 
	{
        $string .= $characters[mt_rand(0, strlen($characters))];
    }

    return $string;
}

// Example
$idhash = genRandomString();

while(alreadyExist($idhash)) // you must make this function yourself of course
{
    $idhash = genRandomString();
}