<?php

   $couponArray = array(
                                 0=>"A",1=>"B",2=>"C",3=>"D",4=>"E",
                                 5=>"F",6=>"G",7=>"H",8=>"I",9=>"J",
                                10=>"K",11=>"L",12=>"M",13=>"N",14=>"O",
                                15=>"P",16=>"Q",17=>"R",18=>"S",19=>"T",
                                20=>"U",21=>"V",22=>"W",23=>"X",24=>"Y",
                                25=>"Z",26=>"0",27=>"1",28=>"2",29=>"3",
                                30=>"4",31=>"5",32=>"6",33=>"7",34=>"8",
                                35=>"9"
                                );

    srand((double)microtime()*1000000); //난수값 초기화

    for($i=0;$i<16;$i++){
          $randNo = rand(0,35); //0과 35사이의 난수를 구한다
          $resultStr .= $couponArray[$randNo];
     }

    echo$resultStr;
?>

<?php

   $couponNo = getCoupon(16);
   echo$couponNo;

function getCoupon($couponLength, $couponString=""){

    $defaultString = "ABCDEFGHIJKLMNOPQRSTUVXYZ0123456789";
    srand((double)microtime()*1000000);

    if ( $couponString == "" ){ //$couponString의 값이 정해지지 않았다면 $defaultString 값으로 사용
           $couponString = $defaultString;
    }

   $length = strlen($couponString);

   for($i=0;$i<$couponLength;$i++)
  {
     $couponStr = rand(0,$length-1); //0에서 $defaultString또는 $couponString의 길이사이의 난수를 구한다
     $resultStr .= substr( $couponString, $couponStr, 1 );
   }

  return $resultStr;
}

?>