php 生成 uuid

function create_guid()
{
$microTime = microtime();
list($a_dec, $a_sec) = explode(" ", $microTime);
$dec_hex = dechex($a_dec* 1000000);
$sec_hex = dechex($a_sec);
ensure_length($dec_hex, 5);
ensure_length($sec_hex, 6);
$guid = "";
$guid .= $dec_hex;
$guid .= create_guid_section(3);
$guid .= '-';
$guid .= create_guid_section(4);
$guid .= '-';
$guid .= create_guid_section(4);
$guid .= '-';
$guid .= create_guid_section(4);
$guid .= '-';
$guid .= $sec_hex;
$guid .= create_guid_section(6);
return $guid;
}
function create_guid_section($characters)
{
$return = "";
for($i=0; $i<$characters; $i++)
{
$return .= dechex(mt_rand(0,15));
}
return $return;
}

function ensure_length(&$string, $length)  

{  
    $strlen = strlen($string);  
   if($strlen < $length)  
   {  
        $string = str_pad($string,$length,"0");  
    }  
     else if($strlen > $length)  
   {  
         $string = substr($string, 0, $length);  
  }  

 }